Equity Financing - Additional Information (Detail) - USD ($)			1 Months Ended				12 Months Ended
	Aug. 17, 2018	Aug. 14, 2018	Feb. 28, 2018	Oct. 31, 2017	Jan. 31, 2017	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 31, 2019	Apr. 19, 2018	Apr. 30, 2017
Subsidiary Sale Of Stock [Line Items]
Proceed from private placement and equity issuances, gross					$ 26,100,000	$ 13,800,000
Preferred stock conversion rate											100.00%
2016 Placement Warrants
Subsidiary Sale Of Stock [Line Items]
Warrants to purchase common stock, shares						1,380,745
Warrant exercise price						$ 2.95	$ 2.95
Warrant expiration period						7 years	7 years
2017 Placement Warrants
Subsidiary Sale Of Stock [Line Items]
Warrants to purchase common stock, shares					1,720,512
Warrant exercise price					$ 3.17		$ 3.17
Warrant expiration date					Jan. 31, 2024
Warrant expiration period							7 years
Warrant exercisable date					Jul. 31, 2017
Underwriters | Common Stock
Subsidiary Sale Of Stock [Line Items]
Underwriting agreement date		Aug. 14, 2018
Number of shares issued in transaction		16,216,217
Sale of stock, price		$ 9.25
Option to purchase additional shares		30 days
Aggregate proceeds from issuance of common stock	$ 161,900,000
Underwriters | Common Stock | Maximum
Subsidiary Sale Of Stock [Line Items]
Number of additional shares issued in transaction		2,432,432
2017 Direct Registered Offering | Common Stock
Subsidiary Sale Of Stock [Line Items]
Aggregate proceeds from issuance of common stock				$ 49,900,000
Stock issued during period				8,382,643
March 2018 Direct Registered Offering
Subsidiary Sale Of Stock [Line Items]
Proceed from private placement and equity issuances, gross			$ 59,100,000
March 2018 Direct Registered Offering | Common Stock
Subsidiary Sale Of Stock [Line Items]
Stock issued during period			4,090,000
March 2018 Direct Registered Offering | Series A Convertible Preferred Stock
Subsidiary Sale Of Stock [Line Items]
Stock issued during period			3,000,581
March 2018 Direct Registered Offering Warrants
Subsidiary Sale Of Stock [Line Items]
Warrants to purchase common stock, shares			1,418,116
Warrant exercise price			$ 8.31
Warrant expiration date			Mar. 31, 2025
2016 Private Placement | Common Stock
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in transaction						4,602,506
Proceed from private placement and equity issuances, gross						$ 13,800,000
2017 Private Placement | Common Stock
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in transaction					8,602,589
Proceed from private placement and equity issuances, gross					$ 26,100,000
At The Market Offering Program
Subsidiary Sale Of Stock [Line Items]
Aggregate proceeds from issuance of common stock							$ 278,000	$ 40,126,000	$ 0
At The Market Offering Program | Maximum
Subsidiary Sale Of Stock [Line Items]
Maximum aggregate offering price					$ 75,000,000
Maximum common stock saleable shares, value												$ 50,000,000
At The Market Offering Program | Maximum | Subsequent Event
Subsidiary Sale Of Stock [Line Items]
Maximum aggregate offering price										$ 250,000,000
Maximum common stock saleable shares, value										$ 100,000,000
At The Market Offering Program | Common Stock
Subsidiary Sale Of Stock [Line Items]
Aggregate proceeds from issuance of common stock							$ 300,000	$ 40,100,000
Stock issued during period							33,097	6,575,062
Average market price per share							$ 8.41	$ 6.10