Business Segments and Geographic Information - Income Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pre-Tax Income Reconciliation of Segments
Total operating income from segments	$ 69,780	$ 62,902	$ 201,181	$ 181,827
Unallocated Amount:
General and Administrative Expenses	(47,647)	(49,074)	(136,927)	(143,866)
Income before income taxes and equity in earnings of affiliates	40,217	38,831	116,596	111,096
Operating Segments
Pre-Tax Income Reconciliation of Segments
Total operating income from segments	117,427	111,976	338,108	325,693
Segment Reconciling Items
Unallocated Amount:
General and Administrative Expenses	(47,647)	(49,074)	(136,927)	(143,866)
Net Interest Expense	$ (29,563)	$ (24,071)	$ (84,585)	$ (70,731)