SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Operations and Comprehensive Income) (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||||||
|---|---|---|---|---|---|---|---|---|
Sep. 30, 2019 |
Jun. 30, 2019 |
Mar. 31, 2019 |
Sep. 30, 2018 |
Jun. 30, 2018 |
Mar. 31, 2018 |
Sep. 30, 2019 |
Sep. 30, 2018 |
|
| Revenues | $ 555.6 | Ìý | Ìý | $ 741.8 | Ìý | Ìý | $ 1,455.8 | $ 1,636.1 |
| COST OF GOODS SOLD | (400.7) | Ìý | Ìý | (480.2) | Ìý | Ìý | (1,007.0) | (1,028.5) |
| SALES MARGIN | 154.9 | Ìý | Ìý | 261.6 | Ìý | Ìý | 448.8 | 607.6 |
| Selling, general and administrative expenses | (25.5) | Ìý | Ìý | (29.1) | Ìý | Ìý | (82.2) | (78.9) |
| Miscellaneous - net | (7.8) | Ìý | Ìý | (7.0) | Ìý | Ìý | (19.0) | (18.7) |
| Other operating expense | (33.3) | Ìý | Ìý | (36.1) | Ìý | Ìý | (101.2) | (97.6) |
| OPERATING INCOME (LOSS) | 121.6 | Ìý | Ìý | 225.5 | Ìý | Ìý | 347.6 | 510.0 |
| Interest expense, net | (25.3) | Ìý | Ìý | (29.5) | Ìý | Ìý | (76.5) | (93.1) |
| Gain (loss) on extinguishment of debt | Ìý | Ìý | Ìý | 0.0 | Ìý | Ìý | (18.2) | 0.2 |
| Other non-operating income (expense) | 0.3 | Ìý | Ìý | 4.3 | Ìý | Ìý | 1.3 | 13.1 |
| TOTAL OTHER INCOME (EXPENSE) | (25.0) | Ìý | Ìý | (25.2) | Ìý | Ìý | (93.4) | (79.8) |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | 96.6 | Ìý | Ìý | 200.3 | Ìý | Ìý | 254.2 | 430.2 |
| Income tax benefit (expense) | (4.8) | Ìý | Ìý | (0.5) | Ìý | Ìý | (23.1) | (14.4) |
| EQUITY IN INCOME OF SUBSIDIARIES | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| INCOME FROM CONTINUING OPERATIONS | 91.8 | Ìý | Ìý | 199.8 | Ìý | Ìý | 231.1 | 415.8 |
| INCOME (LOSS) FROM DISCONTINUED OPERATIONS, NET OF TAX | (0.9) | Ìý | Ìý | 238.0 | Ìý | Ìý | (1.5) | 102.8 |
| NET INCOME (LOSS) | 90.9 | $ 160.8 | $ (22.1) | 437.8 | $ 165.1 | $ (84.3) | 229.6 | 518.6 |
| Other Comprehensive Income (Loss), Net of Tax | 6.2 | 3.7 | 8.4 | (221.2) | $ 9.1 | $ 7.7 | 18.3 | (204.4) |
| TOTAL COMPREHENSIVE INCOME (LOSS) | 97.1 | $ 164.5 | $ (13.7) | 216.6 | Ìý | Ìý | 247.9 | 314.2 |
| Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| COST OF GOODS SOLD | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| SALES MARGIN | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Selling, general and administrative expenses | (20.7) | Ìý | Ìý | (23.1) | Ìý | Ìý | (67.1) | (62.9) |
| Miscellaneous - net | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | (0.4) |
| Other operating expense | (20.7) | Ìý | Ìý | (23.1) | Ìý | Ìý | (67.1) | (63.3) |
| OPERATING INCOME (LOSS) | (20.7) | Ìý | Ìý | (23.1) | Ìý | Ìý | (67.1) | (63.3) |
| Interest expense, net | (24.9) | Ìý | Ìý | (29.2) | Ìý | Ìý | (75.0) | (91.9) |
| Gain (loss) on extinguishment of debt | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | (18.2) | 0.2 |
| Other non-operating income (expense) | (0.9) | Ìý | Ìý | (0.9) | Ìý | Ìý | (3.0) | (2.6) |
| TOTAL OTHER INCOME (EXPENSE) | (25.8) | Ìý | Ìý | (30.1) | Ìý | Ìý | (96.2) | (94.3) |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | (46.5) | Ìý | Ìý | (53.2) | Ìý | Ìý | (163.3) | (157.6) |
| Income tax benefit (expense) | (4.8) | Ìý | Ìý | (0.3) | Ìý | Ìý | (22.7) | (13.8) |
| EQUITY IN INCOME OF SUBSIDIARIES | 142.2 | Ìý | Ìý | 471.0 | Ìý | Ìý | 415.5 | 665.7 |
| INCOME FROM CONTINUING OPERATIONS | 90.9 | Ìý | Ìý | 417.5 | Ìý | Ìý | 229.5 | 494.3 |
| INCOME (LOSS) FROM DISCONTINUED OPERATIONS, NET OF TAX | 0.0 | Ìý | Ìý | 20.3 | Ìý | Ìý | 0.1 | 24.3 |
| NET INCOME (LOSS) | 90.9 | Ìý | Ìý | 437.8 | Ìý | Ìý | 229.6 | 518.6 |
| Other Comprehensive Income (Loss), Net of Tax | 6.2 | Ìý | Ìý | (221.2) | Ìý | Ìý | 18.3 | (204.4) |
| TOTAL COMPREHENSIVE INCOME (LOSS) | 97.1 | Ìý | Ìý | 216.6 | Ìý | Ìý | 247.9 | 314.2 |
| Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 555.6 | Ìý | Ìý | 741.8 | Ìý | Ìý | 1,455.8 | 1,636.1 |
| COST OF GOODS SOLD | (400.7) | Ìý | Ìý | (480.2) | Ìý | Ìý | (1,007.0) | (1,028.5) |
| SALES MARGIN | 154.9 | Ìý | Ìý | 261.6 | Ìý | Ìý | 448.8 | 607.6 |
| Selling, general and administrative expenses | (4.7) | Ìý | Ìý | (5.7) | Ìý | Ìý | (14.7) | (15.3) |
| Miscellaneous - net | (7.7) | Ìý | Ìý | (6.5) | Ìý | Ìý | (17.9) | (16.9) |
| Other operating expense | (12.4) | Ìý | Ìý | (12.2) | Ìý | Ìý | (32.6) | (32.2) |
| OPERATING INCOME (LOSS) | 142.5 | Ìý | Ìý | 249.4 | Ìý | Ìý | 416.2 | 575.4 |
| Interest expense, net | (0.6) | Ìý | Ìý | (0.4) | Ìý | Ìý | (1.9) | (1.8) |
| Gain (loss) on extinguishment of debt | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | 0.0 | 0.0 |
| Other non-operating income (expense) | (3.4) | Ìý | Ìý | 0.1 | Ìý | Ìý | (9.8) | 0.8 |
| TOTAL OTHER INCOME (EXPENSE) | (4.0) | Ìý | Ìý | (0.3) | Ìý | Ìý | (11.7) | (1.0) |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | 138.5 | Ìý | Ìý | 249.1 | Ìý | Ìý | 404.5 | 574.4 |
| Income tax benefit (expense) | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | (0.3) | (0.2) |
| EQUITY IN INCOME OF SUBSIDIARIES | 4.6 | Ìý | Ìý | 4.7 | Ìý | Ìý | 13.3 | 13.8 |
| INCOME FROM CONTINUING OPERATIONS | 143.1 | Ìý | Ìý | 253.8 | Ìý | Ìý | 417.5 | 588.0 |
| INCOME (LOSS) FROM DISCONTINUED OPERATIONS, NET OF TAX | (0.3) | Ìý | Ìý | 12.9 | Ìý | Ìý | (0.3) | 12.8 |
| NET INCOME (LOSS) | 142.8 | Ìý | Ìý | 266.7 | Ìý | Ìý | 417.2 | 600.8 |
| Other Comprehensive Income (Loss), Net of Tax | 6.8 | Ìý | Ìý | 6.1 | Ìý | Ìý | 20.1 | 18.0 |
| TOTAL COMPREHENSIVE INCOME (LOSS) | 149.6 | Ìý | Ìý | 272.8 | Ìý | Ìý | 437.3 | 618.8 |
| Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| COST OF GOODS SOLD | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| SALES MARGIN | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Selling, general and administrative expenses | (0.1) | Ìý | Ìý | (0.3) | Ìý | Ìý | (0.4) | (0.7) |
| Miscellaneous - net | (0.1) | Ìý | Ìý | (0.5) | Ìý | Ìý | (1.1) | (1.4) |
| Other operating expense | (0.2) | Ìý | Ìý | (0.8) | Ìý | Ìý | (1.5) | (2.1) |
| OPERATING INCOME (LOSS) | (0.2) | Ìý | Ìý | (0.8) | Ìý | Ìý | (1.5) | (2.1) |
| Interest expense, net | 0.2 | Ìý | Ìý | 0.1 | Ìý | Ìý | 0.4 | 0.6 |
| Gain (loss) on extinguishment of debt | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | 0.0 | 0.0 |
| Other non-operating income (expense) | 4.6 | Ìý | Ìý | 5.1 | Ìý | Ìý | 14.1 | 14.9 |
| TOTAL OTHER INCOME (EXPENSE) | 4.8 | Ìý | Ìý | 5.2 | Ìý | Ìý | 14.5 | 15.5 |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | 4.6 | Ìý | Ìý | 4.4 | Ìý | Ìý | 13.0 | 13.4 |
| Income tax benefit (expense) | 0.0 | Ìý | Ìý | (0.2) | Ìý | Ìý | (0.1) | (0.4) |
| EQUITY IN INCOME OF SUBSIDIARIES | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| INCOME FROM CONTINUING OPERATIONS | 4.6 | Ìý | Ìý | 4.2 | Ìý | Ìý | 12.9 | 13.0 |
| INCOME (LOSS) FROM DISCONTINUED OPERATIONS, NET OF TAX | (0.6) | Ìý | Ìý | 204.8 | Ìý | Ìý | (1.3) | 65.7 |
| NET INCOME (LOSS) | 4.0 | Ìý | Ìý | 209.0 | Ìý | Ìý | 11.6 | 78.7 |
| Other Comprehensive Income (Loss), Net of Tax | 0.0 | Ìý | Ìý | (230.5) | Ìý | Ìý | 0.0 | (227.5) |
| TOTAL COMPREHENSIVE INCOME (LOSS) | 4.0 | Ìý | Ìý | (21.5) | Ìý | Ìý | 11.6 | (148.8) |
| Consolidation, Eliminations [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| COST OF GOODS SOLD | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| SALES MARGIN | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Selling, general and administrative expenses | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Miscellaneous - net | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Other operating expense | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| OPERATING INCOME (LOSS) | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Interest expense, net | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Gain (loss) on extinguishment of debt | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | 0.0 | 0.0 |
| Other non-operating income (expense) | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| TOTAL OTHER INCOME (EXPENSE) | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Income tax benefit (expense) | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| EQUITY IN INCOME OF SUBSIDIARIES | (146.8) | Ìý | Ìý | (475.7) | Ìý | Ìý | (428.8) | (679.5) |
| INCOME FROM CONTINUING OPERATIONS | (146.8) | Ìý | Ìý | (475.7) | Ìý | Ìý | (428.8) | (679.5) |
| INCOME (LOSS) FROM DISCONTINUED OPERATIONS, NET OF TAX | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| NET INCOME (LOSS) | (146.8) | Ìý | Ìý | (475.7) | Ìý | Ìý | (428.8) | (679.5) |
| Other Comprehensive Income (Loss), Net of Tax | (6.8) | Ìý | Ìý | 224.4 | Ìý | Ìý | (20.1) | 209.5 |
| TOTAL COMPREHENSIVE INCOME (LOSS) | (153.6) | Ìý | Ìý | (251.3) | Ìý | Ìý | (448.9) | (470.0) |
| Product [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 515.0 | Ìý | Ìý | 684.7 | Ìý | Ìý | 1,357.8 | 1,525.9 |
| Product [Member] | Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Product [Member] | Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 515.0 | Ìý | Ìý | 684.7 | Ìý | Ìý | 1,357.8 | 1,525.9 |
| Product [Member] | Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Product [Member] | Consolidation, Eliminations [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Freight [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 40.6 | Ìý | Ìý | 57.1 | Ìý | Ìý | 98.0 | 110.2 |
| Freight [Member] | Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Freight [Member] | Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 40.6 | Ìý | Ìý | 57.1 | Ìý | Ìý | 98.0 | 110.2 |
| Freight [Member] | Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | 0.0 | Ìý | Ìý | 0.0 | Ìý | Ìý | 0.0 | 0.0 |
| Freight [Member] | Consolidation, Eliminations [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Revenues | $ 0.0 | Ìý | Ìý | $ 0.0 | Ìý | Ìý | $ 0.0 | $ 0.0 |