Derivative Instruments and Hedging Activities - Notional principal amounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016
Derivative Instruments and Hedging Activities
Term of derivative contract	3 years
Unrealized gain (loss) on derivative	$ (3,300)	$ (4,900)
Instruments designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	142,209		$ 158,664
Instruments not designated as accounting hedges: | Foreign currency forwards
Derivative Instruments and Hedging Activities
Notional principal outstanding derivative instruments	110,844		115,070
Notional principal outstanding derivative instruments designed to manage exposure	$ 81,100		$ 78,400