Consolidated Statements of Income and Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Foreign currency translation adjustment, income tax (expense) benefit	$ 0	$ 0	$ 1,066
Cash flow hedges, income tax expense	594	545	(91)
Amortization of pension and postretirement plan liabilities, income tax (expense) benefit	$ 0	$ 0	$ 470