Derivative Instruments, Hedging Activities and Fair Value (Details 3) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2022	Dec. 31, 2018	Dec. 31, 2017
Foreign Currency Derivatives
Pre-tax net gains (losses) on certain loans designated as hedges of net investments in foreign subsidiaries	$ (16,100)	$ 7,700	$ (6,600)	$ 9,500
Foreign currency forward exchange contracts
Foreign Currency Derivatives
U.S. Dollar Equivalent	649,565		649,565				$ 671,857
Recognized Gain (Loss)			10,946	(1,879)
Foreign currency forward exchange contracts | British pounds sterling | Sell
Foreign Currency Derivatives
U.S. Dollar Equivalent	96,546		96,546				76,761
Recognized Gain (Loss)			3,024	(769)
Foreign currency forward exchange contracts | British pounds sterling | Buy
Foreign Currency Derivatives
U.S. Dollar Equivalent	9,590		9,590				5,960
Recognized Gain (Loss)			(157)	72
Foreign currency forward exchange contracts | Euros | Sell
Foreign Currency Derivatives
U.S. Dollar Equivalent	306,503		306,503				314,649
Recognized Gain (Loss)			8,258	(4,916)
Foreign currency forward exchange contracts | Euros | Buy
Foreign Currency Derivatives
U.S. Dollar Equivalent	192,929		192,929				223,111
Recognized Gain (Loss)			(1,435)	4,564
Foreign currency forward exchange contracts | Other currencies | Sell
Foreign Currency Derivatives
U.S. Dollar Equivalent	41,096		41,096				39,889
Recognized Gain (Loss)			1,296	(1,049)
Foreign currency forward exchange contracts | Other currencies | Buy
Foreign Currency Derivatives
U.S. Dollar Equivalent	2,901		2,901				$ 11,487
Recognized Gain (Loss)			(40)	$ 219
Term Loan Facility, Fixed-Rate | Term Loan
Foreign Currency Derivatives
Principal amount	$ 300,000		$ 300,000
LIBOR | Scenario, Forecast | Term Loan Facility, Fixed-Rate | Term Loan
Foreign Currency Derivatives
Variable rate basis spread					3.12%	1.65%