Derivative Instruments and Hedging Activities - Notional principal amounts (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2019 USD ($) building	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)
Two Buildings on Existing Campus
Derivative Instruments and Hedging Activities
Unrealized gain (loss) on derivative	$ 1,400
Number of building under lease | building	2
Instruments designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	$ 132,551		$ 143,164
Instruments designated as accounting hedges: | Interest Rate Swaps
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	95,000		95,000
Instruments not designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	57,844		24,220
Notional principal outstanding derivative instruments designed to manage exposure	51,200		$ 14,000
Unrealized gain (loss) on derivative	$ 800	$ 300