Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income		$ 60,336	$ 33,700	$ 121,081	$ 74,865
Other comprehensive income, net of tax:
Amortization of prior service credit included in net income, net of tax benefit of $(565), $(838), $(1,695) and $(2,513)	[1]	(1,608)	(1,309)	(4,824)	(3,929)
Amortization of actuarial loss included in net income, net of tax expense of $1,164, $1,562, $3,491 and $4,686	[1]	3,311	2,443	9,934	7,330
Cash flow hedge, net of tax expense (benefit) of $166, $0, $386 and $(87)		1,591		1,850	(137)
Other comprehensive income, net of tax		3,294	1,134	6,960	3,264
Comprehensive income		$ 63,630	$ 34,834	$ 128,041	$ 78,129

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