Inventories (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Inventories
Materials and supplies	$ 67,461	$ 60,326
Less: Obsolescence allowance	(643)	(586)
Materials and supplies, net	66,818	59,740
Coal inventory	4,830	5,230
Inventories, net	71,648	64,970
Provision to increase the obsolescence allowance and charged inventory costs
Provision to increase the obsolescence allowance	535	976	973
Inventory costs charged to the obsolescence allowance	$ 478	$ 1,709	$ 897