Derivative Instruments and Hedging Activities - Fair value of derivative financial instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Derivative Instruments and Hedging Activities
Asset derivatives	$ 2,385	$ 3,494
Liability derivatives	17,707	757
Foreign currency forwards | Other current assets
Derivative Instruments and Hedging Activities
Asset derivatives	1,761	2,635
Foreign currency forwards | Other assets
Derivative Instruments and Hedging Activities
Asset derivatives	624	619
Foreign currency forwards | Accrued compensation and current liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	4,365	529
Foreign currency forwards | Other noncurrent liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	1,118	228
Forward starting swap | Other assets
Derivative Instruments and Hedging Activities
Asset derivatives		$ 240
Forward starting swap | Other noncurrent liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	$ 12,224