Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)		$ 24,244	$ (31,238)	$ 41,165	$ (31,081)
Other comprehensive income, net of tax:
Amortization of prior service credit included in net periodic cost, net of tax benefit of $(838), $(815), $(1,675) and $(1,630)	[1]	(1,310)	(1,274)	(2,620)	(2,549)
Amortization of actuarial loss included in net periodic cost, net of tax expense of $1,465, $1,826, $3,124 and $3,521	[1]	2,292	2,857	4,887	5,507
Cash flow hedge, net of tax of $(118), $(264), $(87) and $(369)		(185)	(413)	(137)	(577)
Other comprehensive income, net of tax		797	1,170	2,130	2,381
Comprehensive income (loss)		$ 25,041	$ (30,068)	$ 43,295	$ (28,700)

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