Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 17,137	$ 46,148	$ 23,697	$ 60,745
Other comprehensive (loss) income, net of tax:
Amortization of prior service credit included in net income, net of tax benefit of $(561), $(565), $(1,122) and $(1,130)	(1,597)	(1,608)	(3,194)	(3,216)
Amortization of actuarial loss included in net income, net of tax expense of $915, $1,155, $1,886 and $2,327	2,605	3,290	5,368	6,623
Cash flow hedges, net of tax (benefit) expense of $(549), $187, $(913) and $220	(10,417)	1,249	(18,930)	259
Other comprehensive (loss) income, net of tax	(9,409)	2,931	(16,756)	3,666
Comprehensive income	$ 7,728	$ 49,079	$ 6,941	$ 64,411