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]
Unrealized investment gains (losses) arising during the period, tax effects	$ (78)	$ 26	$ 96
Amortization of net actuarial gain (loss), tax effects	653	(1,538)	2,380
Amortization of prior service credit (cost), tax effects	$ (210)	$ 339	$ (327)