Inventories (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Inventory Disclosure [Abstract]
Coal	$ 85,043	$ 71,901
Repair parts and supplies	111,677	118,352
Inventories	196,720	190,253
Inventories
Valuation and Qualifying Accounts Disclosure [Line Items]
Allowance for slow-moving and obsolete inventories	$ 6,000	$ 6,600