Income Taxes (Details 3) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Deferred Tax Assets, Valuation Allowance, Current		$ 49.7	$ 80.0
Deferred tax assets:
Tax credits and loss carryforwards		1,817.4	1,723.5
Accrued postretirement benefit obligations		372.4	372.3
Asset retirement obligations		160.9	167.0
Employee benefits		69.6	70.7
Payable to voluntary employee beneficiary association for certain Patriot retirees	[1]	52.9	79.2
Hedge activities		26.6	44.2
Environmental contingencies		0.0	29.9
Deferred revenue		0.0	29.1
Financial guarantees		16.9	16.9
Workers' compensation obligations		13.7	6.2
Other		66.7	50.5
Total gross deferred tax assets		2,597.1	2,589.5
Deferred tax liabilities:
Property, plant, equipment and mine development, principally due to differences in depreciation, depletion and asset impairments		966.6	1,223.4
Unamortized discount on Convertible Junior Subordinated Debentures		130.3	131.0
Investments and other assets		70.1	73.4
Other		0.0	1.1
Total gross deferred tax liabilities		(1,167.0)	(1,428.9)
Valuation allowance, income tax		(1,447.3)	(1,169.0)
Net deferred tax liability		(17.2)	(8.4)
Deferred taxes are classified as follows:
Noncurrent deferred income taxes		(66.9)	(88.4)
Net deferred tax liability		$ (17.2)	$ (8.4)

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