Derivative Instruments and Hedging Activities - Notional principal amounts (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021 USD ($) building	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) building	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Two Buildings on Existing Campus
Derivative Instruments and Hedging Activities
Unrealized gain (loss) on derivative	$ 2,600	$ (5,000)	$ 2,700	$ (3,600)
Number of building under lease | building	2		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	$ 128,920		$ 128,920		$ 92,931
Instruments receiving hedge accounting treatment | Interest Rate Swaps
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	595,000		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		500,000
Unrealized gain (loss) on derivative	7,300		8,300
Instruments receiving hedge accounting treatment | Interest Rate Swaps | Two Buildings on Existing Campus
Derivative Instruments and Hedging Activities
Notional principal amount	95,000		95,000		95,000
Instruments not receiving hedge accounting treatment | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	4,538		4,538		4,298
Notional principal outstanding derivative instruments designed to manage exposure	3,500		3,500		$ 4,300
Unrealized gain (loss) on derivative	$ (200)	$ (2,600)	$ 600	$ (1,800)