Acquisition (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	12 Months Ended
	Aug. 31, 2012 company	Dec. 31, 2012 Mines	Dec. 31, 2011 T
Recognized identified assets acquired and liabilities assumed
Goodwill		$ 6.4	$ 0
Reed Minerals
Business Acquisition [Line Items]
Number of related companies acquired	4
Number of mines on leased reserves in central alabama		3
Tons of coal sold reported by acquired entity for last annual period			900,000
Revenue reported by acquired entity for last annual period			86.0
Income reported by acquired entity for last annual period			4.0
Payments to Acquire Businesses, Gross		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
Acquisition related expenses		2.6
Recognized identified assets acquired and liabilities assumed
Property, plant and equipment (including mineral rights)	60.2
Other assets	15.0
Identifiable 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
Results of Reed Minerals included in the Company's Consolidated Statements of Operations
Revenues		29.3
Operating profit		1.5
Net income		$ 1.0