Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Tax expense (benefit) on actuarial gain (loss), employee benefit plans	$ (3,507)	$ (5,205)	$ 5,644
Tax expense, adjustment for items reclassified to earnings, employee benefit plans	$ 2,220	$ 1,757	$ 1,507