Derivative Instruments and Hedging Activities - Notional principal amounts (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020 USD ($) building	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) building	Jun. 30, 2019 USD ($)	Apr. 01, 2020 USD ($)	Sep. 30, 2019 USD ($)	Aug. 31, 2019 USD ($)
Two Buildings on Existing Campus
Derivative Instruments and Hedging Activities
Unrealized gain (loss) on derivative	$ 2,500		$ 4,700
Number of building under lease | building	2		2
Maximum
Derivative Instruments and Hedging Activities
Term of derivative contract			5 years
Instruments designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	$ 112,508		$ 112,508			$ 143,164
Instruments designated as accounting hedges: | Interest Rate Swaps
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	645,000		645,000			95,000
Instruments designated as accounting hedges: | Interest Rate Swaps | Credit Facility
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments					$ 550,000
Unrealized gain (loss) on derivative	(7,800)		(7,800)
Instruments designated as accounting hedges: | Interest Rate Swaps | Two Buildings on Existing Campus
Derivative Instruments and Hedging Activities
Notional principal amount							$ 95,000
Instruments not designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	2,218		2,218			24,220
Notional principal outstanding derivative instruments designed to manage exposure	2,200		2,200			$ 14,000
Unrealized gain (loss) on derivative	$ 1,100	$ (200)	$ (600)	$ (300)