Equity Financing - Additional Information (Detail) - USD ($)			1 Months Ended			9 Months Ended
	Aug. 17, 2018	Aug. 14, 2018	Feb. 28, 2018	Oct. 31, 2017	Jan. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Apr. 30, 2017
2017 Placement Warrants
Equity Financing [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 exercisable date					Jul. 31, 2017
Underwriters | Common Stock
Equity Financing [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
Equity Financing [Line Items]
Number of additional shares issued in transaction		2,432,432
2017 Direct Registered Offering
Equity Financing [Line Items]
Aggregate proceeds from issuance of common stock				$ 49,900,000
2017 Direct Registered Offering | Common Stock
Equity Financing [Line Items]
Stock issued during period				8,382,643
March 2018 Direct Registered Offering
Equity Financing [Line Items]
Warrants to purchase common stock, shares			1,418,116
Proceed from private placement and equity issuances, gross			$ 59,100,000
Warrant exercise price			$ 8.31
Warrant expiration date			Mar. 31, 2025
March 2018 Direct Registered Offering | Common Stock
Equity Financing [Line Items]
Stock issued during period			4,090,000
March 2018 Direct Registered Offering | Series A Convertible Preferred Stock
Equity Financing [Line Items]
Stock issued during period			3,000,581
2017 Private Placement | Common Stock
Equity Financing [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
Equity Financing [Line Items]
Aggregate proceeds from issuance of common stock						$ 278,000	$ 40,126,000
Average market price per share						$ 8.41
At The Market Offering Program | Maximum
Equity Financing [Line Items]
Maximum aggregate offering price					$ 75,000,000
Maximum common stock saleable shares, value								$ 50,000,000
At The Market Offering Program | Common Stock
Equity Financing [Line Items]
Stock issued during period						33,097