Derivative Instruments and Hedging Activities - Fair value of derivative financial instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Derivative Instruments and Hedging Activities
Asset derivatives	$ 9,601	$ 3,494
Liability derivatives	6,680	757
Foreign currency forwards | Other current assets
Derivative Instruments and Hedging Activities
Asset derivatives	5,540	2,635
Foreign currency forwards | Other assets
Derivative Instruments and Hedging Activities
Asset derivatives	4,061	619
Foreign currency forwards | Other current liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	1,657	529
Foreign currency forwards | Other noncurrent liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	1,418	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	$ 3,605