Business Segments and Geographic Information - Income Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pre-Tax Income Reconciliation of Segments
Total operating income from segments	$ 66,951	$ 54,553	$ 131,401	$ 118,925
Unallocated Amount:
General and Administrative Expenses	(47,448)	(52,206)	(89,280)	(94,792)
Income before income taxes and equity in earnings of affiliates	38,699	30,916	76,379	72,265
Operating Segments
Pre-Tax Income Reconciliation of Segments
Total operating income from segments	114,399	106,759	220,681	213,717
Segment Reconciling Items
Unallocated Amount:
General and Administrative Expenses	(47,448)	(52,206)	(89,280)	(94,792)
Net Interest Expense	$ (28,252)	$ (23,637)	$ (55,022)	$ (46,660)