Standby Letters of Credit (Details) (USD $)	Jun. 30, 2011
Standby Letters of Credit
Risk management requirement for insurance claims	$ 41,083,000
Payment surety required by utilities	133,000
Contractual reclamation/restoration requirements	8,468,000
Financial requirement for industrial revenue bond	14,230,000
Total	63,914,000
Standby Letters of Credit (Textuals)
Total outstanding standby letters of credit	63,914,000
Amount backed by bank credit facility	60,882,000
Amount of bank credit facility	$ 1,500,000,000