Interest Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Interest expense, net
Senior notes	$ 38,250	$ 40,702	$ 50,250
Credit facility unutilized fee	2,753	2,485	2,842
Federal coal lease obligations imputed interest	2,960	8,062	13,212
Amortization of deferred financing costs and original issue discount	4,174	4,321	4,499
Other	442	260	240
Subtotal	48,579	55,830	71,043
Premium on early retirement of debt	0	13,837	0
Write-off of deferred financing costs and original issue discount	0	7,338	0
Other	0	364	0
Subtotal - cost of early retirement of debt and refinancings	0	21,538	0
Total interest expense	48,579	77,368	71,043
Less interest capitalized	(1,018)	(208)	(29,378)
Interest expense	$ 47,561	$ 77,160	$ 41,665