Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 10,938	$ 31,714	$ 89,910
Other comprehensive income (loss), net of tax:
Net loss arising during the period, net of tax benefit of $(1,826), $(1,913) and $(15,598)	(2,857)	(2,990)	(24,396)
Amortization of actuarial loss included in net periodic cost, net of tax expense of $7,042, $7,794 and $6,488	11,014	12,190	10,149
Amortization of prior service credit included in net periodic cost, net of tax benefit of $(3,260), $(3,396) and $(3,468)	(5,099)	(5,311)	(5,425)
Cash flow hedge, net of tax of $448, $-, and $-	701	0	0
Other comprehensive income (loss), net of tax	3,759	3,889	(19,672)
Comprehensive income	$ 14,697	$ 35,603	$ 70,238