Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 20,565	$ 60,336	$ 44,262	$ 121,081
Other comprehensive (loss) income, net of tax:
Amortization of prior service credit included in net income, net of tax benefit of $(562), $(565), $(1,684) and $(1,695)	(1,598)	(1,608)	(4,792)	(4,824)
Amortization of actuarial loss included in net income, net of tax expense of $943, $1,164, $2,829 and $3,491	2,685	3,311	8,053	9,934
Cash flow hedges, net of tax (benefit) expense of $(387), $166, $(1,300) and $386	(6,978)	1,591	(25,908)	1,850
Other comprehensive (loss) income, net of tax	(5,891)	3,294	(22,647)	6,960
Comprehensive income	$ 14,674	$ 63,630	$ 21,615	$ 128,041