Condensed Consolidated Statement of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income		$ 16,921	$ 157
Other comprehensive income, net of tax:
Amortization of prior service credit included in net periodic cost, net of tax benefit of $(837) and $(815)	[1]	(1,310)	(1,275)
Amortization of actuarial loss included in net periodic cost, net of tax expense of $1,659 and $1,695	[1]	2,595	2,650
Cash flow hedge, net of tax of $31 and $(105)		48	(164)
Other comprehensive income, net of tax		1,333	1,211
Comprehensive income		$ 18,254	$ 1,368

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