Condensed Consolidated Statements of Comprehensive (Loss) Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 6,560	$ 14,597
Other comprehensive (loss) income, net of tax:
Amortization of prior service credit included in net income, net of tax benefit of $(561) and $(565)	(1,597)	(1,608)
Amortization of actuarial loss included in net income, net of tax expense of $971 and $1,172	2,763	3,333
Cash flow hedges, net of tax benefit of $(364) and $(32)	(8,513)	(990)
Other comprehensive (loss) income, net of tax	(7,347)	735
Comprehensive (loss) income	$ (787)	$ 15,332