Resource Management, Acquisitions and Other Commercial Events (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2014	Jun. 30, 2014	Feb. 03, 2012
OtherCommercialEvents [Line Items]
Proceeds from sale of nonstrategic assets	$ 203.7	$ 178.3	$ 147.9
Interest in unincorporated joint venture project	50.00%
Noncontrolling interest, decrease from purchase of interests			(49.8)
MCGH [Member]
OtherCommercialEvents [Line Items]
Equity interest, converted from receivable, per court ruling						90.00%
Business combinations, recognized identifiable assets acquired and liabilities assumed, noncontrolling interest						39.0
Noncontrolling interest, decrease from purchase of interests			49.8
Decrease in APIC due to MCGH NCI purchases			10.8
Nonstrategic Australian mining tenement [Member]
OtherCommercialEvents [Line Items]
Proceeds from sale of nonstrategic assets				62.6
Gain (loss) on sale of nonstrategic asset	0
Nonstrategic Kentucky Coal Reserves And Surface Lands [Member]
OtherCommercialEvents [Line Items]
Proceeds from sale of nonstrategic assets	29.6	70.0
Gain (loss) on sale of nonstrategic asset	13.6	40.3
Western U.S. Mining Segment Customer [Member]
OtherCommercialEvents [Line Items]
Sales-related expenses	6.4
Positive Outcome of Litigation [Member] | Western U.S. Mining Segment Customer [Member]
OtherCommercialEvents [Line Items]
Contract revenue	$ 33.5				$ 43.2