Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($)	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Amount of net losses associated with interest rate hedges to be reclassified with interest rate hedges during the next 12 months	$ (3,500,000)
Amount of net gains associated with currency rate risk to be reclassified into earnings during the next 12 months	(3,200,000)
Restricted cash in brokerage accounts	19,800,000	$ 10,300,000	$ 7,900,000
Foreign currency contracts
Derivative [Line Items]
Notional amount	485,700,000	$ 424,800,000	416,700,000
Net Investment Hedging | Foreign currency contracts
Derivative [Line Items]
Notional amount	$ 0		$ 0