Stockholders' Equity - Summary of Stock Option Transactions (Detail) - Stock Options [Member] - $ / shares	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Shares Available for Grant
Options Available, Beginning balance	1,150,961
Options Available, Additional shares authorized	223,742
Options Available, Granted	(464,000)
Options Available, Exercised	0
Options Available, Canceled/Forfeited	15,046
Options Available, Ending balance	925,749		1,150,961
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance	1,667,153
Options granted	464,000	616,500
Options exercised	0
Options canceled/forfeited	(15,046)
Ending balance	2,116,107		1,667,153
Options vested at end of period (shares)	901,140
Options vested and expected to vest at end of period (shares)	2,116,107
Weighted-Average Exercise Price per Share
Beginning balance (in dollars per share)	$ 6.03
Options granted (in dollars per share)	1.67
Options exercised (in dollars per share)	0
Options canceled/forfeited (in dollars per share)	8.83
Ending balance (in dollars per share)	5.05		$ 6.03
Options vested at end of period (in dollars per share)	9.14
Options vested and expected to vest at end of period (in dollars per share)	$ 5.05
Weighted Average Remaining Contractual Term
Options outstanding at end of period	8 years 5 months 4 days		8 years 3 months 7 days
Options vested at end of period	7 years 6 months 7 days
Options vested and expected to vest at end of period	8 years 5 months 4 days