Stockholders' Equity - Stock Option Activity (Detail) - $ / shares	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options Outstanding, Beginning Balance (in shares)	2,845,866
Granted (in shares)	299,397
Exercised (in shares)	(63,329)
Canceled (in shares)	(19,667)
Options Outstanding, Ending Balance (in shares)	3,062,267	2,845,866
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding, Beginning Balance (in dollars per share)	$ 4.21
Granted (in dollars per share)	8.60
Exercised (in dollars per share)	2.16
Canceled (in dollars per share)	3.89
Outstanding, Ending Balance (in dollars per share)	$ 4.68	$ 4.21
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-Average Remaining Contractual Term, Outstanding (in years)	5 years 7 months 17 days	5 years 5 months 1 day