Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income		$ 46,148	$ 24,244	$ 60,745	$ 41,165
Other comprehensive income, net of tax:
Amortization of prior service credit included in net income, net of tax benefit of $(565), $(838), $(1,130) and $(1,675)	[1]	(1,608)	(1,310)	(3,216)	(2,620)
Amortization of actuarial loss included in net income, net of tax expense of $1,155, $1,465, $2,327 and $3,124	[1]	3,290	2,292	6,623	4,887
Cash flow hedge, net of tax expense (benefit) of $187, $(118), $220 and $(87)		1,249	(185)	259	(137)
Other comprehensive income, net of tax		2,931	797	3,666	2,130
Comprehensive income		$ 49,079	$ 25,041	$ 64,411	$ 43,295

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