Commitments (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 16, 2017 ft²
Operating Leased Assets [Line Items]
Deferred rent charges included in other long-term liabilities	$ 600	$ 1,000
Rent expense	10,800	$ 11,500	$ 12,500
Future minimum payments under the leases
2018	9,432
2019	8,696
2020	4,903
2021	2,806
2022	1,001
2023	305
Total future minimum lease payments	27,143
Other Operating Expense
Operating Leased Assets [Line Items]
Cease use charge	2,000
Office space | Bethesda, Maryland
Operating Leased Assets [Line Items]
Number of square feet | ft²				18,412
Office space | DISTRICT OF COLUMBIA | Other Operating Expense
Operating Leased Assets [Line Items]
Cease use charge	$ 2,000