Stock Option Plan - Additional information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended		12 Months Ended
	Nov. 21, 2019	Aug. 19, 2019	Jul. 22, 2019	Jan. 31, 2019	Sep. 30, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 24, 2021	Dec. 31, 2017	Apr. 26, 2006
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance									31,115,329
Outstanding options issued						6,832,386	5,842,879	5,277,085		3,852,135
Proceeds from stock options exercised						$ 442	$ 1,219	$ 240
Total intrinsic value of options						$ 300	$ 1,100	$ 100
Stock options, granted						2,222,368	2,880,691	1,744,950
Stock options granted exercise price						$ 3.39	$ 3.40	$ 2.35
Share-based payment award granted						805,900	1,519,766
Options available for future grant						5,714,648
Common stock, shares authorized						250,000,000	250,000,000				250,000,000
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of shares of restricted common stock							225,339	514,349
restricted stock in lieu				446,428
Restricted Stock | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of shares of restricted common stock, price per share							$ 4.72	$ 4.41
Restricted Stock | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchase of shares of restricted common stock, price per share							$ 2.24	$ 1.70
Restricted Stock | Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment award granted						805,900	1,519,766
Unvested Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs related to non-vested restricted stock outstanding						$ 7,300
Expected recognition period						1 year 9 months 10 days
Unvested Restricted Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected recognition period						1 year 4 months 2 days
Share-based payment award granted						805,900	1,519,766	150,321
Unrecognized stock-based compensation expense related to non-vested restricted stock arrangements						$ 1,800
the "2012 Plan"
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance						14,000,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized						21
Options available for future grant						22,066,275
Share Based Payment Award Shares Registered Due to Proximity On Authorized Shares						5,750,000
the "2012 Plan" | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance						5,659,018
the "2012 Plan" | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding options issued						57,331,551
the "2012 Plan" | Director | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding options issued						1,039,231
the "2012 Plan" | Consultants | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding options issued						60,000
Outside 2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding options issued						588,333
Stock options, granted	65,000	65,000	400,000		500,000
Stock options granted exercise price	$ 4.59	$ 5.18	$ 5.60		$ 6.16
Share Based compensation arrangement by share based payment award options grants grant date fair value	$ 193	$ 231	$ 1,500		$ 2,200
Options available for future grant						1,066,275
the "2020 Plan" | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance						1,173,368