Stock Option Plan - Additional information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance				5,284,988
Outstanding options issued	3,852,135			5,277,085	3,852,135	3,465,335	3,481,468
Proceeds from stock options exercised				$ 240	$ 88	$ 714
Total intrinsic value of options				$ 100	$ 200	$ 800
Stock options, granted				1,744,950	688,800	362,800
Stock options granted exercise price				$ 2.35	$ 5.27	$ 6.40
Options available for future grant				3,895,923
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of shares of restricted common stock			71,166,676,667	514,349	394,269	133,656
Restricted Stock | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of shares of restricted common stock, price per share				$ 4.41	$ 7.12	$ 7.74
Restricted Stock | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of shares of restricted common stock, price per share				$ 1.70	$ 4.14	$ 5.35
Restricted Stock | Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment award granted				30,000
Restricted Stock | Non Employee Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment award granted	838,000			120,321	69,032	86,020
Unvested Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs related to non-vested restricted stock outstanding				$ 5,100
Expected recognition period				1 year 6 months 11 days
Unvested Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected recognition period				1 year 3 months 29 days
Share-based payment award granted				150,321	907,032	711,770
Unrecognized stock-based compensation expense related to non-vested restricted stock arrangements				$ 2,600
the "2012 Plan"
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance				14,000,000
the "2012 Plan" | Employees | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding options issued				4,146,135
the "2012 Plan" | Director | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding options issued				1,120,950
the "2012 Plan" | Consultants | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding options issued				10,000
Outside 2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance				526,364
Outstanding options issued				500,000
Stock options, granted		500,000
Stock options granted exercise price		$ 6.16
Share Based compensation arrangement by share based payment award options grants grant date fair value		$ 2,200