Derivative Instruments and Hedging Activities - Notional principal amounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Derivative Instruments and Hedging Activities
Term of derivative contract	3 years
Unrealized gain (loss) on derivative	$ 300	$ 3,300
Instruments designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	123,575		$ 125,486
Instruments not designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	43,264		35,117
Notional principal outstanding derivative instruments designed to manage exposure	$ 28,800		$ 18,500