Annual report pursuant to Section 13 and 15(d)

INCOME TAXES (Schedule of Effective Rate Reconciliation) (Details)

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INCOME TAXES (Schedule of Effective Rate Reconciliation) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Income Tax Contingency [Line Items]      
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount $ 65.5 $ 118.6 $ 37.9
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 21.00% 35.00%
Income Tax Reconciliation Tax Deduction for percentage depletion in excess of cost depletion $ (49.3) $ (54.6) $ (61.6)
Income Tax Reconciliation Tax Deduction for percentage depletion in excess of cost depletion percent (15.80%) (9.70%) (56.90%)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount $ 0.0 $ 0.0 $ 407.5
Impact Of Tax Law Change Percent 0.00% 0.00% 376.60%
Effective Income Tax Rate Reconciliation, Disposition of Business, Amount $ 846.0 $ 161.7 $ 0.0
Effective Income Tax Rate Reconciliation, Disposition of Business, Percent 271.10% 28.60% 0.00%
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount $ 0.2 $ 0.1 $ 477.9
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent 0.10% 0.00% 441.70%
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount $ 1.2 $ 0.8 $ (1.5)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent 0.40% 0.20% (1.40%)
Income Tax Expense (Benefit) $ 17.6 $ (475.2) $ (252.4)
Effective Income Tax Rate Reconciliation, Percent 5.70% (84.10%) (233.30%)
Tax Law Change [Member]      
Income Tax Contingency [Line Items]      
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount $ 0.0 $ 0.0 $ (407.5)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent 0.00% 0.00% (376.60%)
Current year activity [Member]      
Income Tax Contingency [Line Items]      
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount $ 0.0 $ (79.6) $ (469.8)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent 0.00% (14.10%) (434.20%)
valuation allowance reversal [Member]      
Income Tax Contingency [Line Items]      
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount $ 0.0 $ (460.5) $ 0.0
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent 0.00% (81.50%) 0.00%
Repeal of AMT [Member]      
Income Tax Contingency [Line Items]      
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount $ 0.0 $ 0.0 $ (235.3)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent 0.00% 0.00% (217.50%)
Dissolution of entities [Member]      
Income Tax Contingency [Line Items]      
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount $ (846.0) $ (161.7) $ 0.0
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent (271.10%) (28.60%) 0.00%