Acquisitions (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011 Tons	Aug. 31, 2012
Business Acquisition [Line Items]
Business Acquisition, Tons Sold Reported by Acquired Entity for Last Annual Period		900,000
Business Acquisition, Revenue Reported by Acquired Entity for Last Annual Period		$ 86.0
Business Acquisition, Income Reported by Acquired Entity for Last Annual Period		4.0
Business Acquisition, Cost of Acquired Entity, Cash Paid			64.8
Business Combination, Contingent Consideration Arrangements, Basis for Amount	average coal selling price received on the first 15 million tons of coal sold by NACoal from the Reed Minerals operations
Property, plant and equipment (including mineral rights)			54.6
Other non-current assets			2.5
Goodwill	7.5	0	7.5
Other intangible assets			15.9
Total assets acquired			80.5
Other current liabilities			0.7
Other long-term liabilities			15.0
Total liabilities assumed			15.7
Net assets acquired			64.8
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	7.7
Business Combination, Pro Form Information, Operating Profit or Loss of Acquiree since Acquisition Date, Actual	0
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	$ 0