Derivative Instruments and Hedging Activities - Fair value of derivative financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Derivative Instruments and Hedging Activities
Asset derivatives	$ 2,406	$ 1,620
Liability derivatives	10,887	18,627
Foreign currency forwards | Other current assets
Derivative Instruments and Hedging Activities
Asset derivatives	2,177	1,398
Foreign currency forwards | Other assets
Derivative Instruments and Hedging Activities
Asset derivatives	229	222
Foreign currency forwards | Accrued compensation and current liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	6,350	4,557
Foreign currency forwards | Other noncurrent liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	1,344	866
Interest Rate Swaps | Other noncurrent liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	$ 3,193	$ 13,204