CONSOLIDATED STATEMENTS OF 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, tax	$ (29,448)	$ 10,977	$ 24,150
Prior service cost recognized in net periodic cost, tax	(39)	(20)	0
Unamortized (loss) gain arising during the period, tax	1,677	1,849	1,469
Plan amendments, tax	74	0	0
Net loss recognized in net periodic cost, tax	(2,457)	(2,489)	(2,242)
Foreign currency translation, tax	413	(373)	(316)
Unrealized gain (loss) on derivatives arising during the period, tax	(631)	1,359	379
Reclassification adjustment on derivatives included in net income, tax	$ 83	$ (1,010)	$ (196)