Derivative Instruments, Hedging Activities and Fair Value - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2022	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Foreign Currency Derivatives
Liabilities	$ 1,356,026,000		$ 1,356,026,000				$ 1,325,152,000	$ 1,363,520,000
Pre-tax net gains (losses) on certain loans designated as hedges of net investments in foreign subsidiaries	(800,000)	$ 7,600,000	(7,500,000)	$ 17,100,000
Fair value of long-term debt	649,000,000.0		649,000,000.0					599,100,000
Carrying value of long-term debt	646,500,000		646,500,000					593,700,000
Subject to Master Netting Agreements
Foreign Currency Derivatives
Net assets	1,300,000		1,300,000
Liabilities								200,000
Foreign currency exchange forward contracts
Foreign Currency Derivatives
U.S. Dollar Equivalent	652,600,000		652,600,000					$ 671,900,000
Term Loan Facility, Fixed-Rate | Term Loan
Foreign Currency Derivatives
Principal amount	$ 300,000,000.0		$ 300,000,000.0
LIBOR | Scenario, Forecast | Term Loan Facility, Fixed-Rate | Term Loan
Foreign Currency Derivatives
Variable rate basis spread					3.12%	1.65%