Derivative Instruments and Hedging Activities - Notional principal amounts (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2020 USD ($) building	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 building	Aug. 31, 2019 USD ($)
Derivative Instruments and Hedging Activities
Number of buildings | building				2
Two Buildings on Existing Campus
Derivative Instruments and Hedging Activities
Unrealized gain (loss) on derivative	$ 100	$ (1,400)
Number of building under lease | building	2
Maximum
Derivative Instruments and Hedging Activities
Term of derivative contract	5 years
Instruments receiving hedge accounting treatment | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	$ 118,922		$ 92,931
Instruments receiving hedge accounting treatment | Interest Rate Swaps
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	595,000		595,000
Instruments receiving hedge accounting treatment | Interest Rate Swaps | Credit Facility
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	500,000		500,000
Unrealized gain (loss) on derivative	1,000
Instruments receiving hedge accounting treatment | Interest Rate Swaps | Two Buildings on Existing Campus
Derivative Instruments and Hedging Activities
Notional principal amount					$ 95,000
Instruments not receiving hedge accounting treatment | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	4,842		4,298
Notional principal outstanding derivative instruments designed to manage exposure	4,800		$ 4,300
Unrealized gain (loss) on derivative	$ 800	$ 800