Commitments - Additional Information (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2016 USD ($) operating_lease	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)
Operating Leased Assets [Line Items]
Number of operating leases | operating_lease	33
Rental expense	$ 4,650	$ 4,426	$ 4,581
Non-cash straight line rent expense	328	$ 48	$ 70
Future minimum lease payments due	30,309
Executive offices | Affiliate of ABP Trust
Operating Leased Assets [Line Items]
Future minimum lease payments due	$ 26,499
Term of lease (in years)	10 years