Warrants - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended	12 Months Ended
	Jan. 31, 2017	Sep. 30, 2016	Aug. 31, 2015	Jul. 31, 2015	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2013
Common Stock Warrants [Line Items]
Proceed from private placement and equity issuances, gross	$ 26,100	$ 13,800
Fair value of warrants upon issuance	$ 3,400	$ 2,700
(Gain) loss related to change in fair value						$ (9,379)	$ 16,598	$ (3)
2018 Direct Registered Offering | Common Stock
Common Stock Warrants [Line Items]
Warrants issued					1,418,116
Exercise price					$ 8.31
Common stock warrants, expiration date					Mar. 31, 2025
Number of warrant exercised						0
Proceed from private placement and equity issuances, gross					$ 59,100
Fair value of warrants upon issuance					$ 7,400
Equity Classified Common Stock Warrants
Common Stock Warrants [Line Items]
Warrants issued									128,231
Exercise price									$ 5.84
Common stock warrants, expiration date						Dec. 16, 2023
Number of warrant exercised						0
2015 Placement Equity Classified Common Stock Warrants | Insiders
Common Stock Warrants [Line Items]
Warrants issued			198,760	198,760
Exercise price			$ 5.00	$ 5.00
Number of warrant exercised						159,010
Warrant expiration period			5 years	5 years
Stock issued during period						92,487
Placement warrants not excercised and remained outstanding						39,750
2016 Placement Warrants
Common Stock Warrants [Line Items]
Warrants issued		1,380,745
Exercise price		$ 2.95				$ 2.95
Warrant expiration period		7 years				7 years
Placement warrants not excercised and remained outstanding						1,130,615	1,355,641
(Gain) loss related to change in fair value						$ (4,000)	$ 7,400
2017 Placement Warrants
Common Stock Warrants [Line Items]
Warrants issued	1,720,512
Exercise price	$ 3.17					$ 3.17
Warrant expiration period						7 years
Placement warrants not excercised and remained outstanding						1,709,532	1,711,123
(Gain) loss related to change in fair value						$ (5,400)	$ 9,200