Income Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Mar. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Income Tax Contingency [Line Items]
Effective tax rate			44.00%	13.00%	59.00%
Release of a FIN 48 reserve related to the federal research and development tax credit			$ 1,219	$ 1,046	$ 784	$ 4,026
Undistributed earnings of foreign subsidiaries			22,200
Operating loss carryforwards, valuation allowance					9,300
Valuation allowance related to deferred tax assets			$ 8,767	8,439
Tax benefit		$ 4,100
Decrease in unrecognized tax benefits due to expiration of certain federal statutes of limitations			11.00%
R&D Tax Credit
Income Tax Contingency [Line Items]
Retroactive tax benefit	$ 800		$ 900		$ 600
Release of a FIN 48 reserve related to the federal research and development tax credit				$ 4,100