SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Operations and Comprehensive Income) (Details) - USD ($) $ in Millions |
3 Months Ended |
6 Months Ended |
Jun. 30, 2018 |
Jun. 30, 2017 |
Jun. 30, 2018 |
Jun. 30, 2017 |
Product |
$ 672.0
|
$ 417.0
|
$ 841.2
|
$ 664.3
|
Freight and venture partners' cost reimbursements |
42.3
|
54.3
|
53.1
|
93.2
|
Revenues |
714.3
|
471.3
|
894.3
|
757.5
|
COST OF GOODS SOLD AND OPERATING EXPENSES |
(429.8)
|
(326.6)
|
(548.3)
|
(563.8)
|
SALES MARGIN |
284.5
|
144.7
|
346.0
|
193.7
|
Selling, general and administrative expenses |
(26.2)
|
(26.6)
|
(51.3)
|
(51.7)
|
Miscellaneous - net |
(4.1)
|
(2.9)
|
(10.2)
|
6.6
|
Other operating expense |
(30.3)
|
(29.5)
|
(61.5)
|
(45.1)
|
OPERATING INCOME |
254.2
|
115.2
|
284.5
|
148.6
|
Interest expense, net |
(31.2)
|
(30.1)
|
(63.6)
|
(71.5)
|
Gain (loss) on extinguishment of debt |
0.2
|
(4.9)
|
0.2
|
(76.8)
|
Other non-operating income |
4.4
|
2.5
|
8.8
|
5.0
|
TOTAL OTHER INCOME (EXPENSE) |
(26.6)
|
(32.5)
|
(54.6)
|
(143.3)
|
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES |
227.6
|
82.7
|
229.9
|
5.3
|
Income Tax Expense (Benefit) |
1.8
|
1.1
|
(13.9)
|
0.0
|
Equity in income (loss) of subsidiaries |
0.0
|
0.0
|
0.0
|
0.0
|
INCOME FROM CONTINUING OPERATIONS |
229.4
|
83.8
|
216.0
|
5.3
|
LOSS FROM DISCONTINUED OPERATIONS, NET OF TAX |
(64.3)
|
(53.7)
|
(135.2)
|
(5.0)
|
NET INCOME |
165.1
|
30.1
|
80.8
|
0.3
|
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST |
0.0
|
1.7
|
0.0
|
3.4
|
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
165.1
|
31.8
|
80.8
|
3.7
|
Other comprehensive income (loss) |
9.1
|
4.9
|
16.8
|
1.9
|
TOTAL COMPREHENSIVE INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
174.2
|
36.7
|
97.6
|
5.6
|
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Product |
0.0
|
0.0
|
0.0
|
0.0
|
Freight and venture partners' cost reimbursements |
0.0
|
0.0
|
0.0
|
0.0
|
Revenues |
0.0
|
0.0
|
0.0
|
0.0
|
COST OF GOODS SOLD AND OPERATING EXPENSES |
0.0
|
0.0
|
0.0
|
0.0
|
SALES MARGIN |
0.0
|
0.0
|
0.0
|
0.0
|
Selling, general and administrative expenses |
(19.7)
|
(19.7)
|
(39.8)
|
(39.2)
|
Miscellaneous - net |
(0.2)
|
(0.8)
|
(0.4)
|
(0.9)
|
Other operating expense |
(19.9)
|
(20.5)
|
(40.2)
|
(40.1)
|
OPERATING INCOME |
(19.9)
|
(20.5)
|
(40.2)
|
(40.1)
|
Interest expense, net |
(30.8)
|
(30.4)
|
(62.7)
|
(72.0)
|
Gain (loss) on extinguishment of debt |
0.2
|
(4.9)
|
0.2
|
(76.8)
|
Other non-operating income |
(0.8)
|
(1.0)
|
(1.7)
|
(2.0)
|
TOTAL OTHER INCOME (EXPENSE) |
(31.4)
|
(36.3)
|
(64.2)
|
(150.8)
|
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES |
(51.3)
|
(56.8)
|
(104.4)
|
(190.9)
|
Income Tax Expense (Benefit) |
2.1
|
8.0
|
(13.5)
|
0.2
|
Equity in income (loss) of subsidiaries |
210.4
|
129.6
|
194.7
|
243.0
|
INCOME FROM CONTINUING OPERATIONS |
161.2
|
80.8
|
76.8
|
52.3
|
LOSS FROM DISCONTINUED OPERATIONS, NET OF TAX |
3.9
|
(49.0)
|
4.0
|
(48.6)
|
NET INCOME |
Ìý
|
31.8
|
Ìý
|
3.7
|
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST |
Ìý
|
0.0
|
Ìý
|
0.0
|
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
165.1
|
31.8
|
80.8
|
3.7
|
Other comprehensive income (loss) |
9.1
|
5.0
|
16.8
|
2.0
|
TOTAL COMPREHENSIVE INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
174.2
|
36.8
|
97.6
|
5.7
|
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Product |
672.0
|
417.0
|
841.2
|
664.3
|
Freight and venture partners' cost reimbursements |
42.3
|
54.3
|
53.1
|
93.2
|
Revenues |
714.3
|
471.3
|
894.3
|
757.5
|
COST OF GOODS SOLD AND OPERATING EXPENSES |
(429.8)
|
(326.6)
|
(548.3)
|
(563.8)
|
SALES MARGIN |
284.5
|
144.7
|
346.0
|
193.7
|
Selling, general and administrative expenses |
(5.4)
|
(5.7)
|
(9.7)
|
(10.1)
|
Miscellaneous - net |
(3.6)
|
(3.7)
|
(8.9)
|
(9.2)
|
Other operating expense |
(9.0)
|
(9.4)
|
(18.6)
|
(19.3)
|
OPERATING INCOME |
275.5
|
135.3
|
327.4
|
174.4
|
Interest expense, net |
(0.6)
|
0.0
|
(1.4)
|
0.0
|
Gain (loss) on extinguishment of debt |
0.0
|
0.0
|
0.0
|
0.0
|
Other non-operating income |
0.2
|
(0.8)
|
0.7
|
(1.6)
|
TOTAL OTHER INCOME (EXPENSE) |
(0.4)
|
(0.8)
|
(0.7)
|
(1.6)
|
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES |
275.1
|
134.5
|
326.7
|
172.8
|
Income Tax Expense (Benefit) |
(0.1)
|
0.8
|
(0.2)
|
0.0
|
Equity in income (loss) of subsidiaries |
4.6
|
3.4
|
9.1
|
6.6
|
INCOME FROM CONTINUING OPERATIONS |
279.6
|
138.7
|
335.6
|
179.4
|
LOSS FROM DISCONTINUED OPERATIONS, NET OF TAX |
(0.3)
|
0.7
|
(0.1)
|
0.9
|
NET INCOME |
Ìý
|
139.4
|
Ìý
|
180.3
|
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST |
Ìý
|
1.7
|
Ìý
|
3.4
|
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
279.3
|
141.1
|
335.5
|
183.7
|
Other comprehensive income (loss) |
6.0
|
5.7
|
11.9
|
16.5
|
TOTAL COMPREHENSIVE INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
285.3
|
146.8
|
347.4
|
200.2
|
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Product |
0.0
|
0.0
|
0.0
|
0.0
|
Freight and venture partners' cost reimbursements |
0.0
|
0.0
|
0.0
|
0.0
|
Revenues |
0.0
|
0.0
|
0.0
|
0.0
|
COST OF GOODS SOLD AND OPERATING EXPENSES |
0.0
|
0.0
|
0.0
|
0.0
|
SALES MARGIN |
0.0
|
0.0
|
0.0
|
0.0
|
Selling, general and administrative expenses |
(1.1)
|
(1.2)
|
(1.8)
|
(2.4)
|
Miscellaneous - net |
(0.3)
|
1.6
|
(0.9)
|
16.7
|
Other operating expense |
(1.4)
|
0.4
|
(2.7)
|
14.3
|
OPERATING INCOME |
(1.4)
|
0.4
|
(2.7)
|
14.3
|
Interest expense, net |
0.2
|
0.3
|
0.5
|
0.5
|
Gain (loss) on extinguishment of debt |
0.0
|
0.0
|
0.0
|
0.0
|
Other non-operating income |
5.0
|
4.3
|
9.8
|
8.6
|
TOTAL OTHER INCOME (EXPENSE) |
5.2
|
4.6
|
10.3
|
9.1
|
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES |
3.8
|
5.0
|
7.6
|
23.4
|
Income Tax Expense (Benefit) |
(0.2)
|
(7.7)
|
(0.2)
|
(0.2)
|
Equity in income (loss) of subsidiaries |
0.0
|
0.0
|
0.0
|
0.0
|
INCOME FROM CONTINUING OPERATIONS |
3.6
|
(2.7)
|
7.4
|
23.2
|
LOSS FROM DISCONTINUED OPERATIONS, NET OF TAX |
(67.9)
|
(5.4)
|
(139.1)
|
42.7
|
NET INCOME |
Ìý
|
(8.1)
|
Ìý
|
65.9
|
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST |
Ìý
|
0.0
|
Ìý
|
0.0
|
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
(64.3)
|
(8.1)
|
(131.7)
|
65.9
|
Other comprehensive income (loss) |
2.2
|
(1.5)
|
3.0
|
(19.3)
|
TOTAL COMPREHENSIVE INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
(62.1)
|
(9.6)
|
(128.7)
|
46.6
|
Consolidation, Eliminations [Member] |
Ìý
|
Ìý
|
Ìý
|
Ìý
|
Product |
0.0
|
0.0
|
0.0
|
0.0
|
Freight and venture partners' cost reimbursements |
0.0
|
0.0
|
0.0
|
0.0
|
Revenues |
0.0
|
0.0
|
0.0
|
0.0
|
COST OF GOODS SOLD AND OPERATING EXPENSES |
0.0
|
0.0
|
0.0
|
0.0
|
SALES MARGIN |
0.0
|
0.0
|
0.0
|
0.0
|
Selling, general and administrative expenses |
0.0
|
0.0
|
0.0
|
0.0
|
Miscellaneous - net |
0.0
|
0.0
|
0.0
|
0.0
|
Other operating expense |
0.0
|
0.0
|
0.0
|
0.0
|
OPERATING INCOME |
0.0
|
0.0
|
0.0
|
0.0
|
Interest expense, net |
0.0
|
0.0
|
0.0
|
0.0
|
Gain (loss) on extinguishment of debt |
0.0
|
0.0
|
0.0
|
0.0
|
Other non-operating income |
0.0
|
0.0
|
0.0
|
0.0
|
TOTAL OTHER INCOME (EXPENSE) |
0.0
|
0.0
|
0.0
|
0.0
|
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES |
0.0
|
0.0
|
0.0
|
0.0
|
Income Tax Expense (Benefit) |
0.0
|
0.0
|
0.0
|
0.0
|
Equity in income (loss) of subsidiaries |
(215.0)
|
(133.0)
|
(203.8)
|
(249.6)
|
INCOME FROM CONTINUING OPERATIONS |
(215.0)
|
(133.0)
|
(203.8)
|
(249.6)
|
LOSS FROM DISCONTINUED OPERATIONS, NET OF TAX |
0.0
|
0.0
|
0.0
|
0.0
|
NET INCOME |
Ìý
|
(133.0)
|
Ìý
|
(249.6)
|
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST |
Ìý
|
0.0
|
Ìý
|
0.0
|
NET INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
(215.0)
|
(133.0)
|
(203.8)
|
(249.6)
|
Other comprehensive income (loss) |
(8.2)
|
(4.3)
|
(14.9)
|
2.7
|
TOTAL COMPREHENSIVE INCOME ATTRIBUTABLE TO CLIFFS SHAREHOLDERS |
$ (223.2)
|
$ (137.3)
|
$ (218.7)
|
$ (246.9)
|