Equity Transactions - Preferred and Common Stock Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Nov. 30, 2018	Feb. 28, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	May 31, 2018	Apr. 30, 2018	May 31, 2017	Apr. 30, 2017
Class of Stock [Line Items]
Dividends and deemed dividends on preferred stock				$ 80	$ 4,350
Proceeds from common stock offering				64,170	0
Stock issuance costs				$ 168	$ 0
Common stock authorized (in shares)				101,500,000	81,500,000	101,500,000	81,500,000	81,500,000	41,500,000
Series N Preferred Stock
Class of Stock [Line Items]
Stock Issued (in shares)			22,500
Proceeds from issuance of preferred stock			$ 45,000
Underwriting commissions and discounts, and other offering costs			$ 2,300
Number of shares converted (in shares)			21,925
Conversion price (in USD per share)			$ 3.0
Dividends and deemed dividends on preferred stock					$ 4,400
Preferred stock outstanding (in shares)				0	575
Preferred stock stated value (in USD per share)				$ 0.001	$ 0.001
Series O Preferred Stock
Class of Stock [Line Items]
Preferred stock outstanding (in shares)				12,575	0
Preferred stock stated value (in USD per share)				$ 0.001	$ 0.001
Common Stock
Class of Stock [Line Items]
Stock Issued (in shares)		23,000,000		23,000,000
Number of shares issued in conversion (in shares)				(8,000,000)	14,616,000
Stock issued, purchase price (in USD per share)		$ 3
Proceeds from common stock offering		$ 69,000
Stock issuance costs		$ 4,800
Common Stock | Series N Preferred Stock
Class of Stock [Line Items]
Number of shares issued in conversion (in shares)			14,600,000
BVF Partners, L.P. | Series N Preferred Stock
Class of Stock [Line Items]
Number of shares converted (in shares)		575
Preferred stock outstanding (in shares)					575
BVF Partners, L.P. | Series O Preferred Stock
Class of Stock [Line Items]
Number of shares issued in conversion (in shares)		12,575
Conversion price (in USD per share)		$ 3
Dividends and deemed dividends on preferred stock				$ 100
Preferred stock outstanding (in shares)				12,575
Preferred stock stated value (in USD per share)				$ 2,000
BVF Partners, L.P. | Common Stock
Class of Stock [Line Items]
Stock Issued (in shares)		6,300,000
Number of shares converted (in shares)		8,000,000
BVF Partners, L.P. | Common Stock | Series O Preferred Stock
Class of Stock [Line Items]
Outstanding preferred convertible into common stock (in shares)				8,400,000
Cowen And Company, LLC | Common Stock
Class of Stock [Line Items]
Stock Issued (in shares)				0
Expected proceeds from common stock offering, net of issuance costs	$ 50,000
Percent of gross proceeds, underwriter commission	3.00%