CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income including non-controlling interests	$ 70,535	$ 106,111	$ 200,214	$ 223,290
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on derivatives designated and qualifying as cash flow hedges, net of tax of $390 and $415 in the three and nine months ended September 30, 2018; $239 and $(95) in the three and nine months ended September 30, 2017.	1,416	(684)	1,039	563
Defined benefit pension plan activity, net of tax of $1,278 and $1,927 in the three and nine months ended September 30, 2018; $2,170 and $2,532 in the three and nine months ended September 30, 2017.	3,855	3,958	5,863	5,384
Currency translation adjustment	(467)	18,931	(31,422)	72,820
Other comprehensive income (loss):	4,804	22,205	(24,520)	78,767
Comprehensive income	75,339	128,316	175,694	302,057
Comprehensive income (loss) attributable to non-controlling interests	(65)	16	(105)	47
Comprehensive income attributable to shareholders	$ 75,404	$ 128,300	$ 175,799	$ 302,010