Derivative Instruments and Hedging Activities - Notional principal amounts (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($) building
Derivative Instruments and Hedging Activities
Number of buildings | building		2
Instruments designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	$ 92,931	$ 143,164
Instruments designated as accounting hedges: | Interest Rate Swaps
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	595,000	95,000
Instruments designated as accounting hedges: | Interest Rate Swaps | Credit Facility
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	500,000
Unrealized gain (loss) on derivative	(8,400)
Instruments designated as accounting hedges: | Interest Rate Swaps | Two Buildings on Existing Campus
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	95,000	95,000
Unrealized gain (loss) on derivative	(5,000)	200
Instruments not designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	4,298	24,220
Notional principal outstanding derivative instruments designed to manage exposure	4,300	14,000
Unrealized gain (loss) on derivative	$ 0	$ 0