Commitments and Contingencies (Details 1) $ in Thousands, t in Millions	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 05, 2014 t	Feb. 27, 2013 Item
Wild Earth Guardians Regulatory Challenge to Office of Surface Minings Approval Process for Mine Plans Litigation [Member]
Commitment and Contingency [Line Items]
Number of different coal mines located in four different states for which approvals are challenged				7
Number of different states where seven different coal mines are located for which approvals are challenged				4
Estimated Quantity of Mineral Rights Under Challenge | t			15.8
Wild Earth Guardians Regulatory Challenge to Office of Surface Minings Approval Process for Mine Plans Litigation [Member] | WYOMING
Commitment and Contingency [Line Items]
Number of challenged approvals issued to subsidiaries				1
Number of challenged approvals transferred to U.S. District Court in Wyoming or Montana relating to other OSM approvals				1
Wild Earth Guardians Regulatory Challenge to Office of Surface Minings Approval Process for Mine Plans Litigation [Member] | MONTANA
Commitment and Contingency [Line Items]
Number of challenged approvals issued to subsidiaries				1
Number of challenged approvals transferred to U.S. District Court in Wyoming or Montana relating to other OSM approvals				1
Equipment
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment | $	$ 20,241	$ 11,751
Land
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment | $	23,678	24,663
Coal purchase commitments
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment | $	2,110	2,592
Transportation agreements
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment | $	649,419	691,530
Materials and supplies
Commitment and Contingency [Line Items]
Outstanding amount of long-term purchase commitment | $	$ 10,290	$ 12,185