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Income Taxes Deferred tax (Details 3) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Deferred tax liabilities:    
Depreciation and amortization $ 317.3 $ 864.2
Depletion 390.8 260.9
Partnership tax basis differences 64.6 67.6
Undistributed earnings of non-U.S. subsidiaries 15.0 15.0
Other liabilities 10.3 150.6
Total deferred tax liabilities 798.0 1,358.3
Before valuation allowance    
Alternative minimum tax credit carryforwards 76.5 46.8
Capital loss carryforwards 3.0 0.1
Foreign tax credit carryforwards 493.5 322.9
Net operating loss carryforwards 408.9 112.0
Pension plans and other benefits 33.4 2.1
Asset retirement obligations 187.6 174.1
Deferred revenue 0.0 252.0
Other assets 388.8 169.7
Subtotal 1,591.7 1,079.7
Valuation allowance 1,530.5 584.1
Net deferred tax assets 61.2 495.6
Net deferred tax liabilities (736.8) (862.7)
Deferred tax assets of both foreign and U.S. jurisdictions $ 361.6 $ 440.3