Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 55,661	$ 122,880	$ 86,453
Other comprehensive (loss) income, net of tax:
Net (loss) gain arising during the period, net of tax (benefit) expense of $(1,348), $(5,521) and $3,990	(3,836)	(15,714)	11,355
Amortization of actuarial loss included in net periodic cost, net of tax expense of $3,772, $4,654 and $6,248	10,737	13,246	9,773
Amortization of prior service credit included in net periodic cost, net of tax benefit of $(2,244), $(2,259) and $(3,350)	(6,389)	(6,432)	(5,239)
Cash flow hedges, net of tax (benefit) expense of $(978), $(119) and $3	(18,440)	(2,415)	4
Other comprehensive (loss) income, net of tax	(17,928)	(11,315)	15,893
Comprehensive income	$ 37,733	$ 111,565	$ 102,346