Quarterly report pursuant to Section 13 or 15(d)

SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details)

v3.8.0.1
SUPPLEMENTARY GUARANTOR INFORMATION (Supplementary Statements of Condensed Consolidating Financial Position) (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Jan. 01, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Cash and cash equivalents $ 786.6 $ 1,007.7 $ 1,007.7 $ 295.3  
Accounts receivable, net 47.2 217.2 140.6    
Inventories 324.4 132.0 183.4    
Supplies and other inventories 81.7 93.9 93.9    
Derivative assets 93.6 51.0 39.4    
Loans to and accounts receivable from the Canadian Entities 50.4 51.6 51.6    
Other current assets 28.5 28.0 28.0    
Assets, Current 1,412.4 1,581.4 1,544.6    
PROPERTY, PLANT AND EQUIPMENT, NET 1,047.3 1,051.0 1,051.0    
Deposits for property, plant and equipment 74.1 17.8 17.8    
Income Taxes Receivable 219.9   235.3    
Investment in subsidiaries 0.0   0.0    
Long term intercompany notes 0.0   0.0    
Other non-current assets 109.2 104.7 104.7    
Total Other Assets 403.2 357.8 357.8    
Assets 2,862.9 2,990.2 2,953.4    
Accounts payable 99.5 129.1 127.7    
Accrued expenses 94.4 107.1 107.1    
Accrued interest 28.2 31.4 31.4    
Contingent claims 54.3 55.6 55.6    
Partnership distribution payable 44.2 44.2 44.2    
Other current liabilities 104.3 87.6 86.2    
Liabilities, Current 424.9 455.0 452.2    
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES 251.4 257.7 257.7    
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 181.2 196.5 196.5    
LONG-TERM DEBT 2,308.2 2,304.2 2,304.2    
Long term intercompany notes payable 0.0   0.0    
OTHER LIABILITIES 182.0 186.9 186.9    
Liabilities 3,347.7 3,400.3 3,397.5    
Stockholders' Equity Attributable to Parent (485.0) (410.3) (444.3)    
Stockholders' Equity Attributable to Noncontrolling Interest 0.2 0.2 0.2    
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (484.8) (410.1) (444.1) $ (703.0) $ (1,330.5)
Liabilities and Equity 2,862.9 $ 2,990.2 2,953.4    
Consolidation, Eliminations [Member]          
Cash and cash equivalents 0.0   0.0    
Accounts receivable, net (2.1)   (0.7)    
Inventories 0.0   0.0    
Supplies and other inventories 0.0   0.0    
Derivative assets 0.0   0.0    
Loans to and accounts receivable from the Canadian Entities 0.0   0.0    
Other current assets 0.0   0.0    
Assets, Current (2.1)   (0.7)    
PROPERTY, PLANT AND EQUIPMENT, NET 0.0   0.0    
Deposits for property, plant and equipment 0.0   0.0    
Income Taxes Receivable 0.0   0.0    
Investment in subsidiaries (1,213.1)   (1,054.2)    
Long term intercompany notes (242.0)   (242.0)    
Other non-current assets 0.0   0.0    
Total Other Assets (1,455.1)   (1,296.2)    
Assets (1,457.2)   (1,296.9)    
Accounts payable (2.1)   (0.7)    
Accrued expenses 0.0   0.0    
Accrued interest 0.0   0.0    
Contingent claims 0.0   0.0    
Partnership distribution payable 0.0   0.0    
Other current liabilities 0.0   0.0    
Liabilities, Current (2.1)   (0.7)    
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES 0.0   0.0    
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 0.0   0.0    
LONG-TERM DEBT 0.0   0.0    
Long term intercompany notes payable (242.0)   (242.0)    
OTHER LIABILITIES 0.0   0.0    
Liabilities (244.1)   (242.7)    
Stockholders' Equity Attributable to Parent (1,213.1)   (1,054.2)    
Stockholders' Equity Attributable to Noncontrolling Interest 0.0   0.0    
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (1,213.1)   (1,054.2)    
Liabilities and Equity (1,457.2)   (1,296.9)    
Reportable Legal Entities [Member] | Non-Guarantor Subsidiaries [Member]          
Cash and cash equivalents 32.0   56.7    
Accounts receivable, net 15.9   33.9    
Inventories 21.2   45.0    
Supplies and other inventories 0.3   5.1    
Derivative assets 0.0   1.5    
Loans to and accounts receivable from the Canadian Entities 0.0   0.0    
Other current assets 5.1   4.1    
Assets, Current 74.5   146.3    
PROPERTY, PLANT AND EQUIPMENT, NET 74.2   74.5    
Deposits for property, plant and equipment 72.2   16.5    
Income Taxes Receivable 0.0   0.0    
Investment in subsidiaries 0.0   0.0    
Long term intercompany notes 242.0   242.0    
Other non-current assets 2.4   5.2    
Total Other Assets 316.6   263.7    
Assets 465.3   484.5    
Accounts payable 25.2   31.6    
Accrued expenses 23.3   28.2    
Accrued interest 0.0   0.0    
Contingent claims 0.0   0.0    
Partnership distribution payable 0.0   0.0    
Other current liabilities 39.2   20.6    
Liabilities, Current 87.7   80.4    
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES (244.3)   (239.3)    
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 38.1   55.9    
LONG-TERM DEBT 0.0   0.0    
Long term intercompany notes payable 0.0   0.0    
OTHER LIABILITIES 21.6   24.0    
Liabilities (96.9)   (79.0)    
Stockholders' Equity Attributable to Parent 562.0   563.3    
Stockholders' Equity Attributable to Noncontrolling Interest 0.2   0.2    
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 562.2   563.5    
Liabilities and Equity 465.3   484.5    
Reportable Legal Entities [Member] | Guarantor Subsidiaries [Member]          
Cash and cash equivalents 1.0   2.1    
Accounts receivable, net 27.7   102.9    
Inventories 303.2   138.4    
Supplies and other inventories 81.4   88.8    
Derivative assets 93.6   37.9    
Loans to and accounts receivable from the Canadian Entities 6.9   6.9    
Other current assets 7.9   7.5    
Assets, Current 521.7   384.5    
PROPERTY, PLANT AND EQUIPMENT, NET 956.9   959.0    
Deposits for property, plant and equipment 1.9   1.3    
Income Taxes Receivable 0.0   0.0    
Investment in subsidiaries 27.4   29.9    
Long term intercompany notes 0.0   0.0    
Other non-current assets 97.9   91.7    
Total Other Assets 127.2   122.9    
Assets 1,605.8   1,466.4    
Accounts payable 71.8   89.7    
Accrued expenses 60.0   59.9    
Accrued interest 0.0   0.0    
Contingent claims 0.0   0.0    
Partnership distribution payable 44.2   44.2    
Other current liabilities 63.3   63.5    
Liabilities, Current 239.3   257.3    
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES 429.6   430.6    
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 143.1   140.6    
LONG-TERM DEBT 0.0   0.0    
Long term intercompany notes payable 0.0   0.0    
OTHER LIABILITIES 142.9   147.2    
Liabilities 954.9   975.7    
Stockholders' Equity Attributable to Parent 650.9   490.7    
Stockholders' Equity Attributable to Noncontrolling Interest 0.0   0.0    
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest 650.9   490.7    
Liabilities and Equity 1,605.8   1,466.4    
Reportable Legal Entities [Member] | Cliffs Shareholders Equity [Member]          
Cash and cash equivalents 753.6   948.9    
Accounts receivable, net 5.7   4.5    
Inventories 0.0   0.0    
Supplies and other inventories 0.0   0.0    
Derivative assets 0.0   0.0    
Loans to and accounts receivable from the Canadian Entities 43.5   44.7    
Other current assets 15.5   16.4    
Assets, Current 818.3   1,014.5    
PROPERTY, PLANT AND EQUIPMENT, NET 16.2   17.5    
Deposits for property, plant and equipment 0.0   0.0    
Income Taxes Receivable 219.9   235.3    
Investment in subsidiaries 1,185.7   1,024.3    
Long term intercompany notes 0.0   0.0    
Other non-current assets 8.9   7.8    
Total Other Assets 1,414.5   1,267.4    
Assets 2,249.0   2,299.4    
Accounts payable 4.6   7.1    
Accrued expenses 11.1   19.0    
Accrued interest 28.2   31.4    
Contingent claims 54.3   55.6    
Partnership distribution payable 0.0   0.0    
Other current liabilities 1.8   2.1    
Liabilities, Current 100.0   115.2    
PENSION AND POSTEMPLOYMENT BENEFIT LIABILITIES 66.1   66.4    
ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS 0.0   0.0    
LONG-TERM DEBT 2,308.2   2,304.2    
Long term intercompany notes payable 242.0   242.0    
OTHER LIABILITIES 17.5   15.7    
Liabilities 2,733.8   2,743.5    
Stockholders' Equity Attributable to Parent (484.8)   (444.1)    
Stockholders' Equity Attributable to Noncontrolling Interest 0.0   0.0    
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest (484.8)   (444.1)    
Liabilities and Equity $ 2,249.0   $ 2,299.4