Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 122,880	$ 86,453	$ 10,938
Other comprehensive (loss) income, net of tax:
Net (loss) gain arising during the period, net of tax (benefit) expense of $(5,521), $3,990 and $(1,826)	(15,714)	11,355	(2,857)
Amortization of actuarial loss included in net periodic cost, net of tax expense of $4,654, $6,248 and $7,042	13,246	9,773	11,014
Amortization of prior service credit included in net periodic cost, net of tax benefit of $(2,260), $(3,350) and $(3,260)	(6,432)	(5,239)	(5,099)
Cash flow hedges, net of tax (benefit) expense of $(119), $3, and $448	(2,415)	4	701
Other comprehensive (loss) income, net of tax	(11,315)	15,893	3,759
Comprehensive income	$ 111,565	$ 102,346	$ 14,697