Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Tax benefit on prior service costs, employee benefit plans	$ 543
Tax expense (benefit) on actuarial gain (loss), employee benefit plans	131	(3,507)	(5,205)
Tax expense, adjustment for items reclassified to earnings, employee benefit plans	$ 2,335	$ 2,220	$ 1,757