Inventories (Inventories) (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Inventories [Abstract]
Coal	$ 267,600,000	$ 206,517,000
Repair parts and supplies	178,626,000	163,527,000
Work-in-process	8,865,000	7,446,000
Inventories	455,091,000	377,490,000
Allowance for slow-moving and obsolete inventories	$ 11,300,000	$ 13,100,000