Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)		$ 33,700	$ 27,646	$ 74,865	$ (3,435)
Other comprehensive income, net of tax:
Amortization of prior service credit included in net periodic cost, net of tax benefit of $(838), $(815), $(2,513) and $(2,445)	[1]	(1,309)	(1,275)	(3,929)	(3,824)
Amortization of actuarial loss included in net periodic cost, net of tax expense of $1,562, $1,760, $4,686 and $5,281	[1]	2,443	2,753	7,330	8,260
Cash flow hedge, net of tax of $0, $72, $(87) and $(297)			112	(137)	(465)
Other comprehensive income, net of tax		1,134	1,590	3,264	3,971
Comprehensive income		$ 34,834	$ 29,236	$ 78,129	$ 536

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