Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Inventory Disclosure [Abstract]
Coal	$ 99,691	$ 71,901
Repair parts and supplies	124,238	118,352
Inventories	223,929	190,253
Allowance for slow-moving and obsolete inventories	$ 6,600	$ 6,600