Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($)	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
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	(3,700,000)
Restricted cash in brokerage accounts	10,600,000	$ 10,300,000	$ 300,000
Foreign currency contracts
Derivative [Line Items]
Notional amount	496,200,000	$ 424,800,000	321,800,000
Net Investment Hedging | Foreign currency contracts
Derivative [Line Items]
Notional amount	$ 0		$ 0