Derivative Instruments and Hedging Activities - Fair value of derivative financial instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Derivative Instruments and Hedging Activities
Asset derivatives	$ 2,859	$ 3,719
Liability derivatives	3,044	4,584
Foreign currency forwards | Other current assets
Derivative Instruments and Hedging Activities
Asset derivatives	2,416	2,591
Foreign currency forwards | Other noncurrent assets
Derivative Instruments and Hedging Activities
Asset derivatives	443	1,128
Foreign currency forwards | Other current liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	2,601	3,456
Foreign currency forwards | Other noncurrent liabilities
Derivative Instruments and Hedging Activities
Liability derivatives	$ 443	$ 1,128