Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 17,644	$ 29,459	$ 48,938	$ 31,981
Other comprehensive income (loss):
Unrealized gain (loss) on derivatives (net of tax of $1 and $(2) for the three months ended June 30, 2018 and 2017, respectively, and $2 and $(23) for the six months ended June 30, 2018 and 2017, respectively)	4	(3)	6	(39)
Foreign currency translation adjustment (net of tax of $(7) and zero for the three months ended June 30, 2018 and 2017, respectively, and $(9) and zero for the six months ended June 30, 2018 and 2017, respectively)	(544)	0	(541)	0
Pension and other post-retirement benefits liability adjustment (net of tax of $799 and $(1,185) for the three months ended June 30, 2018 and 2017, respectively, and $1,529 and $(845) for the six months ended June 30, 2018 and 2017, respectively)	2,505	(1,967)	4,798	(1,402)
Comprehensive income	$ 19,609	$ 27,489	$ 53,201	$ 30,540