Summary of Significant Accounting Polices - EPS (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / shares	Sep. 30, 2017 USD ($) item $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) contract $ / shares shares
Revenue Recognition
Number of estimates, a change in which could have material effect on financial position or results of operations | item									1
Allocation of arrangement consideration to the up-front deliverables | $									$ 0
Revenue for the delivery of up-front units of accounting | $									$ 0
Number of transportation systems service contracts, which contain annual system usage incentives | contract											1
Stock-Based Compensation
Vesting period of performance-based RSUs									3 years
Net Income Per Share
Anti-dilutive employee share-based awards									400
Net income (loss) | $	$ 13,155	$ (21,957)	$ 461	$ (2,868)	$ (7,493)	$ 4,498	$ 10,144	$ (5,414)	$ (11,209)	$ 1,735	$ 22,885
Weighted average shares - basic									27,106	26,976	26,872
Effect of dilutive securities (in shares)										64	66
Weighted average shares - diluted									27,106	27,040	26,938
Basic (in dollars per share) | $ / shares	$ 0.49	$ (0.81)	$ 0.02	$ (0.11)	$ (0.29)	$ 0.17	$ 0.38	$ (0.20)	$ (0.41)	$ 0.06	$ 0.85
Diluted (in dollars per share) | $ / shares	$ 0.49	$ (0.81)	$ 0.02	$ (0.11)	$ (0.29)	$ 0.17	$ 0.38	$ (0.20)	$ (0.41)	$ 0.06	$ 0.85