Equity Financing - Additional Information (Detail) - USD ($)	1 Months Ended					6 Months Ended		12 Months Ended
	Feb. 28, 2018	Oct. 31, 2017	Jan. 31, 2017	Sep. 30, 2016	Aug. 31, 2016	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Apr. 30, 2017
2016 Placement Warrants
Subsidiary Sale Of Stock [Line Items]
Warrants to purchase common stock, shares				1,380,745	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
Proceed from private placement and equity issuances, gross			$ 26,100,000
Warrant exercisable date			Jul. 31, 2017
Warrant expiration date			Jan. 31, 2024
At The Market Offering Program
Subsidiary Sale Of Stock [Line Items]
Aggregate gross proceeds from issuance of common stock						$ 278,000	$ 39,524,000	$ 40,100,000
Compensation percentage pertaining to gross sales price per share sold						3.00%
Average market price per share						$ 8.41		$ 6.10
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			$ 25,000,000						$ 25,000,000
At The Market Offering Program | Common Stock
Subsidiary Sale Of Stock [Line Items]
Stock issued during period						33,097		6,575,062
2016 Private Placement | Common Stock
Subsidiary Sale Of Stock [Line Items]
Private placement, common shares				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]
Private placement, common shares			8,602,589
Proceed from private placement and equity issuances, gross			$ 26,100,000
2017 Direct Registered Offering
Subsidiary Sale Of Stock [Line Items]
Aggregate gross proceeds from issuance of common stock		$ 49,900,000
2017 Direct Registered Offering | Common Stock
Subsidiary Sale Of Stock [Line Items]
Stock issued during period		8,382,643
2018 Direct Registered Offering
Subsidiary Sale Of Stock [Line Items]
Warrants to purchase common stock, shares	1,418,116
Warrant exercise price	$ 8.31
Proceed from private placement and equity issuances, gross	$ 59,100,000
Warrant expiration date	Mar. 31, 2025
2018 Direct Registered Offering | Common Stock
Subsidiary Sale Of Stock [Line Items]
Stock issued during period	4,090,000
2018 Direct Registered Offering | Series A Convertible Preferred Stock
Subsidiary Sale Of Stock [Line Items]
Stock issued during period	3,000,581