Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income		$ 14,597	$ 16,921
Other comprehensive income, net of tax:
Amortization of prior service credit included in net income, net of tax benefit of $(565) and $(837)	[1]	(1,608)	(1,310)
Amortization of actuarial loss included in net income, net of tax expense of $1,172 and $1,659	[1]	3,333	2,595
Cash flow hedge, net of tax (benefit) expense of $(32) and $31		(990)	48
Other comprehensive income, net of tax		735	1,333
Comprehensive income		$ 15,332	$ 18,254

[1] Amortization of prior service credit (cost) and amortization of actuarial loss are included in the computation of net periodic cost (benefit).