Income Taxes (Details 2) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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									$ (696.6)	$ (191.7)	$ (257.0)
Changes in valuation allowance, income tax									462.0	569.4	(29.4)
Changes in tax reserves									(21.4)	(81.5)	8.8
Excess depletion									(53.7)	(65.3)	(72.7)
Foreign earnings repatriation									0.0	(71.4)	0.0
Foreign earnings provision differential									146.5	28.8	62.7
General business tax credits									(15.7)	(19.2)	(18.9)
Minerals resource rent tax, net of federal tax									0.0	16.1	(87.4)
Remeasurement of foreign income tax accounts	$ 0.5	$ (0.8)		$ (0.2)	$ 1.2	$ (1.2)	$ (1.3)	$ (1.4)	(0.5)	(2.7)	(44.3)
State income taxes, net of federal tax benefit									(20.1)	(2.3)	(0.2)
Other, net									23.1	21.0	(9.9)
Income tax (benefit) provision	$ 7.9		$ 67.4						$ (176.4)	$ 201.2	$ (448.3)