Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 86,453	$ 10,938	$ 31,714
Other comprehensive income, net of tax:
Net gain (loss) arising during the period, net of tax expense (benefit) of $3,990, $(1,826) and $(1,913)	11,355	(2,857)	(2,990)
Amortization of actuarial loss included in net periodic cost, net of tax expense of $6,248, $7,042 and $7,794	9,773	11,014	12,190
Amortization of prior service credit included in net periodic cost, net of tax benefit of $(3,350), $(3,260) and $(3,396)	(5,239)	(5,099)	(5,311)
Cash flow hedge, net of tax of $3, $448, and $-	4	701	0
Other comprehensive income, net of tax	15,893	3,759	3,889
Comprehensive income	$ 102,346	$ 14,697	$ 35,603