Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net actuarial loss arising in period, tax	$ 3,391	$ 568	$ (14,227)
Amortization of prior service cost included in pension expense, tax	4	6	6
Amortization of actuarial loss included in pension expense, tax	1,301	1,755	1,032
Losses arising in period, tax	(9)	(26)	0
Reclassifications to income in period, tax	$ 28	$ 8	$ 7