Derivative Instruments and Hedging Activities - Notional principal amounts (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Derivative Instruments and Hedging Activities
Term of derivative contract			3 years
Unrealized gain (loss) on derivative	$ 200	$ 1,800	$ (200)	$ (1,900)
Instruments designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	128,877		128,877		$ 125,486
Instruments not designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	30,078		30,078		35,117
Notional principal outstanding derivative instruments designed to manage exposure	$ 19,200		$ 19,200		$ 18,500