Income Taxes (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets:
Tax credits and loss carryforwards	$ 1,390.1	$ 432.8
Minerals resource rent tax	689.7	0
Postretirement benefit obligations	473.3	485.4
Intangible tax asset and purchased contract rights	7.9	13.4
Accrued reclamation and mine closing liabilities	137.9	112.9
Accrued long-term workers’ compensation liabilities	15.7	15.6
Employee benefits	64.7	53.3
Financial guarantees	18.6	18.5
Other	56.5	57.8
Total gross deferred tax assets	2,854.4	1,189.7
Deferred tax liabilities:
Property, plant, equipment and mine development, leased coal interests and advance royalties, principally due to differences in depreciation, depletion and asset writedowns	1,596.9	1,318.0
Unamortized discount on Convertible Junior Subordinated Debentures	132.8	132.5
Hedge activities	37.5	45.5
Investments and other assets	126.3	109.8
Total gross deferred tax liabilities	1,893.5	1,605.8
Valuation allowance, income tax	(714.9)	(79.8)
Valuation allowance mineral resource rent tax	(766.9)	0
Net deferred tax liability	(520.9)	(495.9)
Deferred taxes are classified as follows:
Current deferred income taxes	56.4	27.3
Noncurrent deferred income taxes	(577.3)	(523.2)
Net deferred tax liability	$ (520.9)	$ (495.9)