Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Before Tax [Abstract]
Foreign currency translation adjustment	$ 270	$ (202)	$ 25
Reclassification of net losses (gains) on derivative instruments	614	613	(2,169)
Postretirement medical liability adjustment	346	1,257	317
Other comprehensive income (loss)	1,230	1,668	(1,827)
Other Comprehensive Income (Loss), Tax [Abstract]
Foreign currency translation adjustment	0	0	0
Reclassification of net losses (gains) on derivative instruments	(116)	(242)	831
Postretirement medical liability adjustment	(133)	(484)	(122)
Other comprehensive income (loss)	(249)	(726)	709
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Foreign currency translation adjustment	270	(202)	25
Reclassification of net losses (gains) on derivative instruments	498	371	(1,338)
Postretirement medical liability adjustment	213	773	195
Other comprehensive income (loss)	981	942	$ (1,118)
Accumulated Other Comprehensive Income, Net of Tax [Abstract]
Foreign currency translation	1,448	1,178
Derivative instruments designated as cash flow hedges	(11,633)	(9,981)
Postretirement medical plans	251	31
Accumulated other comprehensive loss	$ (9,934)	$ (8,772)