Acquisitions (Details) (Reed Minerals, USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended
	Aug. 31, 2012	Mar. 31, 2013
Reed Minerals
Business Acquisition [Line Items]
Payments to acquire business	$ 69.3
Contingent consideration	4.0
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
Liability for contingent consideration		4.4
Earn-out payments		0.1
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]
Property, plant and equipment (including mineral rights)	60.2
Other assets	15.0
Other intangible assets	8.2
Total assets acquired	83.4
Other current liabilities	8.3
Other long-term liabilities	8.2
Total liabilities assumed	16.5
Net assets acquired	66.9
Purchase price	73.3
Goodwill	$ 6.4