Income Taxes (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Deferred tax assets:
Tax credits and loss carryforwards	$ 1,723.5		$ 1,558.0
Minerals resource rent tax	0		993.2
Accrued postretirement benefit obligations	372.3		309.9
Asset retirement obligations	167.0		141.6
Employee benefits	70.7		88.5
Payable to voluntary employee beneficiary association for certain Patriot retirees	79.2	[1]	84.4	[1]
Hedge activities	44.2		0
Environmental contingencies	29.9		23.6
Deferred revenue	29.1		3.2
Financial guarantees	16.9		16.9
Workers' compensation obligations	6.2		10.4
Other	50.5		31.5
Total gross deferred tax assets	2,589.5		3,261.2
Deferred tax liabilities:
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments	1,223.4		1,322.3
Unamortized discount on Convertible Junior Subordinated Debentures	131.0		131.8
Hedge activities	0		11.4
Investments and other assets	73.4		82.0
Other	1.1		46.3
Total gross deferred tax liabilities	1,428.9		1,593.8
Valuation allowance, income tax	(1,169.0)		(656.4)
Valuation allowance, mineral resource rent tax	0		(977.7)
Net deferred tax (liability) asset	(8.4)		33.3
Deferred taxes are classified as follows:
Current deferred income taxes	80.0		66.4
Noncurrent deferred income taxes	(88.4)		(33.1)
Net deferred tax (liability) asset	$ (8.4)		$ 33.3

[1] Refer to Note 25. "Matters Related to the Bankruptcy Reorganization of Patriot Coal Corporation" herein for additional details related to this transaction.