Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net actuarial loss arising in period, tax	$ 568	$ (14,227)	$ 7,783
Amortization of prior service cost included in pension expense, tax	6	6	6
Amortization of actuarial loss included in pension expense, tax	1,755	1,032	2,015
Amortization of transition obligation included in pension expense, tax	0	0	1
Gains (losses) arising in period, tax	(26)	0	0
Reclassifications to income in period, tax	$ 8	$ 7	$ 14