Income Taxes (Details 2) - USD ($) $ in Millions	3 Months Ended							9 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of the expected statutory federal income tax provision to the Company's actual income tax provision
Expected income tax benefit at U.S. federal statutory rate									$ (265.4)	$ (696.6)	$ (191.7)
Changes in valuation allowance, income tax									2,453.9	452.9	534.7
Worthless Partnership Deduction									(2,204.4)	0.0	0.0
Changes in tax reserves									2.3	(21.4)	(81.5)
Excess depletion									(37.2)	(53.7)	(65.3)
Foreign earnings repatriation									0.0	0.0	(71.4)
Foreign earnings provision differential									27.5	146.5	28.8
General business tax credits									(14.2)	(15.7)	(19.2)
Minerals resource rent tax, net of federal tax									0.0	0.0	16.1
Remeasurement of foreign income tax accounts				$ 0.5	$ (0.8)		$ (0.2)		(2.0)	(22.1)	(21.8)
State income taxes, net of federal tax benefit									(90.2)	(20.1)	(2.3)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Restructuring Charges, Percent									29.6
Reorganization Items	$ 33.9	$ 29.7	$ 95.4					$ 159.0	159.0	0.0	0.0
Other, net									5.6	23.1	21.0
Income tax (benefit) provision				$ 7.9		$ 67.4		$ (22.7)	$ (94.5)	$ (207.1)	$ 147.4