Derivative Instruments and Hedging Activities - Fair value of derivative financial instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Derivative Instruments and Hedging Activities
Asset derivatives	$ 2,213	$ 1,620
Liability derivatives	20,068	18,627
Foreign currency forwards | Other current assets
Derivative Instruments and Hedging Activities
Asset derivatives	1,443	1,398
Foreign currency forwards | Other assets
Derivative Instruments and Hedging Activities
Asset derivatives	770	222
Foreign currency forwards | Accrued compensation and current liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	6,055	4,557
Foreign currency forwards | Other noncurrent liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	1,968	866
Interest Rate Swaps | Other noncurrent liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	$ 12,045	$ 13,204