Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Interest expense, net
Senior notes	$ 9,563	$ 9,563	$ 19,125	$ 21,577
Credit facility unutilized fee	649	562	1,384	1,223
Federal coal lease obligations imputed interest	1,488	2,612	2,847	5,224
Amortization of deferred financing costs and original issue discount	1,030	1,027	2,094	2,163
Other	45	68	93	145
Subtotal	12,775	13,832	25,543	30,332
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	16	0	364
Total cost of early retirement of debt and refinancings	0	16	0	21,538
Total interest expense	12,775	13,848	25,543	51,870
Less interest capitalized	(154)	(35)	(254)	(63)
Net interest expense	$ 12,621	$ 13,813	$ 25,289	$ 51,807