SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Operations and Comprehensive Income) (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||||
|---|---|---|---|---|---|---|
Jun. 30, 2019 |
Mar. 31, 2019 |
Jun. 30, 2018 |
Mar. 31, 2018 |
Jun. 30, 2019 |
Jun. 30, 2018 |
|
| Product Revenue | $ 697.4 | Ìý | $ 672.0 | Ìý | $ 842.8 | $ 841.2 |
| Freight Revenue | 45.8 | Ìý | 42.3 | Ìý | 57.4 | 53.1 |
| Revenues | 743.2 | Ìý | 714.3 | Ìý | 900.2 | 894.3 |
| COST OF GOODS SOLD | (480.2) | Ìý | (429.8) | Ìý | (606.3) | (548.3) |
| SALES MARGIN | 263.0 | Ìý | 284.5 | Ìý | 293.9 | 346.0 |
| Selling, general and administrative expenses | (30.6) | Ìý | (26.2) | Ìý | (58.7) | (51.3) |
| Miscellaneous - net | (5.6) | Ìý | (4.1) | Ìý | (9.2) | (10.2) |
| Other operating expense | (36.2) | Ìý | (30.3) | Ìý | (67.9) | (61.5) |
| OPERATING INCOME (LOSS) | 226.8 | Ìý | 254.2 | Ìý | 226.0 | 284.5 |
| Interest expense, net | (26.1) | Ìý | (31.2) | Ìý | (51.2) | (63.6) |
| Gain (loss) on extinguishment of debt | (17.9) | Ìý | 0.2 | Ìý | (18.2) | 0.2 |
| Other non-operating income (expense) | 0.6 | Ìý | 4.4 | Ìý | 1.0 | 8.8 |
| TOTAL OTHER INCOME (EXPENSE) | (43.4) | Ìý | (26.6) | Ìý | (68.4) | (54.6) |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | 183.4 | Ìý | 227.6 | Ìý | 157.6 | 229.9 |
| Income tax benefit (expense) | 22.0 | Ìý | (1.8) | Ìý | 18.3 | 13.9 |
| EQUITY IN INCOME OF SUBSIDIARIES | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| INCOME FROM CONTINUING OPERATIONS | 161.4 | Ìý | 229.4 | Ìý | 139.3 | 216.0 |
| Income (loss) from discontinued operations, net of tax | (0.6) | Ìý | (64.3) | Ìý | (0.6) | (135.2) |
| NET INCOME (LOSS) | 160.8 | $ (22.1) | 165.1 | $ (84.3) | 138.7 | 80.8 |
| Other Comprehensive Income (Loss), Net of Tax | 3.7 | 8.4 | 9.1 | $ 7.7 | 12.1 | 16.8 |
| TOTAL COMPREHENSIVE INCOME (LOSS) | 164.5 | $ (13.7) | 174.2 | Ìý | 150.8 | 97.6 |
| Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Product Revenue | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Freight Revenue | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Revenues | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| COST OF GOODS SOLD | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| SALES MARGIN | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Selling, general and administrative expenses | (23.6) | Ìý | (19.7) | Ìý | (46.4) | (39.8) |
| Miscellaneous - net | 0.0 | Ìý | (0.2) | Ìý | 0.0 | (0.4) |
| Other operating expense | (23.6) | Ìý | (19.9) | Ìý | (46.4) | (40.2) |
| OPERATING INCOME (LOSS) | (23.6) | Ìý | (19.9) | Ìý | (46.4) | (40.2) |
| Interest expense, net | (25.4) | Ìý | (30.8) | Ìý | (50.1) | (62.7) |
| Gain (loss) on extinguishment of debt | (17.9) | Ìý | 0.2 | Ìý | (18.2) | 0.2 |
| Other non-operating income (expense) | (1.0) | Ìý | (0.8) | Ìý | (2.1) | (1.7) |
| TOTAL OTHER INCOME (EXPENSE) | (44.3) | Ìý | (31.4) | Ìý | (70.4) | (64.2) |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | (67.9) | Ìý | (51.3) | Ìý | (116.8) | (104.4) |
| Income tax benefit (expense) | 21.8 | Ìý | (2.1) | Ìý | 17.9 | 13.5 |
| EQUITY IN INCOME OF SUBSIDIARIES | 250.4 | Ìý | 210.4 | Ìý | 273.3 | 194.7 |
| INCOME FROM CONTINUING OPERATIONS | 160.7 | Ìý | 161.2 | Ìý | 138.6 | 76.8 |
| Income (loss) from discontinued operations, net of tax | 0.1 | Ìý | 3.9 | Ìý | 0.1 | 4.0 |
| NET INCOME (LOSS) | 160.8 | Ìý | 165.1 | Ìý | 138.7 | 80.8 |
| Other Comprehensive Income (Loss), Net of Tax | 3.7 | Ìý | 9.1 | Ìý | 12.1 | 16.8 |
| TOTAL COMPREHENSIVE INCOME (LOSS) | 164.5 | Ìý | 174.2 | Ìý | 150.8 | 97.6 |
| Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Product Revenue | 697.4 | Ìý | 672.0 | Ìý | 842.8 | 841.2 |
| Freight Revenue | 45.8 | Ìý | 42.3 | Ìý | 57.4 | 53.1 |
| Revenues | 743.2 | Ìý | 714.3 | Ìý | 900.2 | 894.3 |
| COST OF GOODS SOLD | (480.2) | Ìý | (429.8) | Ìý | (606.3) | (548.3) |
| SALES MARGIN | 263.0 | Ìý | 284.5 | Ìý | 293.9 | 346.0 |
| Selling, general and administrative expenses | (6.8) | Ìý | (6.4) | Ìý | (12.0) | (11.1) |
| Miscellaneous - net | (4.8) | Ìý | (3.6) | Ìý | (8.2) | (8.9) |
| Other operating expense | (11.6) | Ìý | (10.0) | Ìý | (20.2) | (20.0) |
| OPERATING INCOME (LOSS) | 251.4 | Ìý | 274.5 | Ìý | 273.7 | 326.0 |
| Interest expense, net | (0.8) | Ìý | (0.6) | Ìý | (1.3) | (1.4) |
| Gain (loss) on extinguishment of debt | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Other non-operating income (expense) | (3.2) | Ìý | 0.2 | Ìý | (6.4) | 0.7 |
| TOTAL OTHER INCOME (EXPENSE) | (4.0) | Ìý | (0.4) | Ìý | (7.7) | (0.7) |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | 247.4 | Ìý | 274.1 | Ìý | 266.0 | 325.3 |
| Income tax benefit (expense) | 0.2 | Ìý | 0.1 | Ìý | 0.3 | 0.2 |
| EQUITY IN INCOME OF SUBSIDIARIES | 4.3 | Ìý | 4.6 | Ìý | 8.7 | 9.1 |
| INCOME FROM CONTINUING OPERATIONS | 251.5 | Ìý | 278.6 | Ìý | 274.4 | 334.2 |
| Income (loss) from discontinued operations, net of tax | 0.1 | Ìý | (0.3) | Ìý | 0.0 | (0.1) |
| NET INCOME (LOSS) | 251.6 | Ìý | 278.3 | Ìý | 274.4 | 334.1 |
| Other Comprehensive Income (Loss), Net of Tax | 6.6 | Ìý | 6.0 | Ìý | 13.3 | 11.9 |
| TOTAL COMPREHENSIVE INCOME (LOSS) | 258.2 | Ìý | 284.3 | Ìý | 287.7 | 346.0 |
| Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Product Revenue | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Freight Revenue | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Revenues | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| COST OF GOODS SOLD | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| SALES MARGIN | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Selling, general and administrative expenses | (0.2) | Ìý | (0.1) | Ìý | (0.3) | (0.4) |
| Miscellaneous - net | (0.8) | Ìý | (0.3) | Ìý | (1.0) | (0.9) |
| Other operating expense | (1.0) | Ìý | (0.4) | Ìý | (1.3) | (1.3) |
| OPERATING INCOME (LOSS) | (1.0) | Ìý | (0.4) | Ìý | (1.3) | (1.3) |
| Interest expense, net | 0.1 | Ìý | 0.2 | Ìý | 0.2 | 0.5 |
| Gain (loss) on extinguishment of debt | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Other non-operating income (expense) | 4.8 | Ìý | 5.0 | Ìý | 9.5 | 9.8 |
| TOTAL OTHER INCOME (EXPENSE) | 4.9 | Ìý | 5.2 | Ìý | 9.7 | 10.3 |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | 3.9 | Ìý | 4.8 | Ìý | 8.4 | 9.0 |
| Income tax benefit (expense) | 0.0 | Ìý | 0.2 | Ìý | 0.1 | 0.2 |
| EQUITY IN INCOME OF SUBSIDIARIES | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| INCOME FROM CONTINUING OPERATIONS | 3.9 | Ìý | 4.6 | Ìý | 8.3 | 8.8 |
| Income (loss) from discontinued operations, net of tax | (0.8) | Ìý | (67.9) | Ìý | (0.7) | (139.1) |
| NET INCOME (LOSS) | 3.1 | Ìý | (63.3) | Ìý | 7.6 | (130.3) |
| Other Comprehensive Income (Loss), Net of Tax | 0.0 | Ìý | 2.2 | Ìý | 0.0 | 3.0 |
| TOTAL COMPREHENSIVE INCOME (LOSS) | 3.1 | Ìý | (61.1) | Ìý | 7.6 | (127.3) |
| Consolidation, Eliminations [Member] | Ìý | Ìý | Ìý | Ìý | Ìý | Ìý |
| Product Revenue | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Freight Revenue | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Revenues | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| COST OF GOODS SOLD | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| SALES MARGIN | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Selling, general and administrative expenses | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Miscellaneous - net | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Other operating expense | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| OPERATING INCOME (LOSS) | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Interest expense, net | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Gain (loss) on extinguishment of debt | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Other non-operating income (expense) | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| TOTAL OTHER INCOME (EXPENSE) | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| INCOME (LOSS) FROM CONTINUING OPERATIONS BEFORE INCOME TAXES | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| Income tax benefit (expense) | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| EQUITY IN INCOME OF SUBSIDIARIES | (254.7) | Ìý | (215.0) | Ìý | (282.0) | (203.8) |
| INCOME FROM CONTINUING OPERATIONS | (254.7) | Ìý | (215.0) | Ìý | (282.0) | (203.8) |
| Income (loss) from discontinued operations, net of tax | 0.0 | Ìý | 0.0 | Ìý | 0.0 | 0.0 |
| NET INCOME (LOSS) | (254.7) | Ìý | (215.0) | Ìý | (282.0) | (203.8) |
| Other Comprehensive Income (Loss), Net of Tax | (6.6) | Ìý | (8.2) | Ìý | (13.3) | (14.9) |
| TOTAL COMPREHENSIVE INCOME (LOSS) | $ (261.3) | Ìý | $ (223.2) | Ìý | $ (295.3) | $ (218.7) |