Equity Financing - Additional Information (Detail) - USD ($)			1 Months Ended			3 Months Ended	9 Months Ended
	Aug. 17, 2018	Aug. 14, 2018	Feb. 28, 2018	Jan. 31, 2017	Sep. 30, 2016	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 31, 2019	Dec. 31, 2018	Apr. 30, 2017
Subsidiary Sale Of Stock [Line Items]
Common stock sold							$ 984,000			$ 952,000
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 exercisable date				Jul. 31, 2017
Warrant expiration period				7 years
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
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
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
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
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
At The Market Offering Program
Subsidiary Sale Of Stock [Line Items]
Aggregate proceeds from issuance of common stock							$ 0	$ 278,000
Common stock sold						$ 300,000	$ 40,400,000	$ 300,000
At The Market Offering Program | Maximum
Subsidiary Sale Of Stock [Line Items]
Maximum aggregate offering price				$ 75,000,000					$ 250,000,000
Maximum common stock saleable shares, value									$ 100,000,000		$ 50,000,000
At The Market Offering Program | Common Stock
Subsidiary Sale Of Stock [Line Items]
Stock issued during period						18,648,649		18,681,746