Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Mine closing reserves	$ 29.0	$ 18.2
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	19.1	19.0
Liabilities incurred during the period	9.0
Liabilities settled during the period	(1.5)	(0.8)
Accretion expense	2.2	1.3
Revision of estimated cash flows	2.7	(0.4)
Balance at end of period	31.5	19.1
Bellaire
Segment Reporting Information [Line Items]
Mine closing reserves	5.0
Reclamation and mine shutdown provision		2.5	2.5
Fair value of trust assets	5.4
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	13.6	13.9
Liabilities incurred during the period	0
Liabilities settled during the period	(1.5)	(0.8)
Accretion expense	1.6	0.9
Revision of estimated cash flows	2.7	(0.4)
Balance at end of period	16.4	13.6	13.9
NACoal
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	5.5	5.1
Liabilities incurred during the period	9.0
Liabilities settled during the period	0	0
Accretion expense	0.6	0.4
Revision of estimated cash flows	0	0
Balance at end of period	$ 15.1	$ 5.5