Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Interest expense, net
Senior notes	$ 9,563	$ 9,563	$ 28,688	$ 31,140
Credit facility unutilized fee	643	614	2,027	1,836
Federal coal lease obligations imputed interest	35	1,478	2,883	6,702
Amortization of deferred financing costs and original issue discount	1,040	1,021	3,133	3,185
Other	44	64	137	209
Subtotal	11,325	12,740	36,868	43,072
Premium on early retirement of debt	0	0	0	13,837
Write-off of deferred financing costs and original issue discount			0	7,338
Other	0	0	0	364
Total cost of early retirement of debt and refinancings	0	0	0	21,538
Total interest expense	11,325	12,740	36,868	64,610
Less interest capitalized	(340)	(39)	(594)	(102)
Net interest expense	$ 10,985	$ 12,701	$ 36,274	$ 64,508