Stockholders' Equity - Common Stock - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended		37 Months Ended
	Jul. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Apr. 24, 2017	Mar. 31, 2017
Class Of Stock [Line Items]
Common stock reserved for future issuance		18,691,478		18,691,478		18,691,478
Issuance of common stock from stock options exercises, net, Shares		0	39,005	500,000	245,271
Issuance of common stock		109,322,626		109,322,626		109,322,626		104,551,466
Issuance of common stock from warrant exercises, net, Shares		0	207,500	0	330,604
Warrants exercised		0	207,500	0	367,500
Issuances of common stock from stock option exercises, Shares		0	39,005	500,000	245,271
Equity Offering Sales Agreement [Member]
Class Of Stock [Line Items]
Equity sales agreement expiration date	Mar. 17, 2018
Issuance of common stock from stock options exercises, net, Shares		2,255,541	0	3,793,758	997,181
Value of shares sold under equity distribution agreement		$ 3.1	$ 0.0	$ 7.1	$ 4.5	$ 17.9
Issuance of common stock		5,790,939		5,790,939		5,790,939
Additional common stocks under shelf registration		$ 14.6		$ 14.6		$ 14.6
Equity Offering Sales Agreement [Member] | Maximum [Member]
Class Of Stock [Line Items]
Common stock reserved for future issuance	26.6
Incentive Award Agreements [Member]
Class Of Stock [Line Items]
Common shares authorized to be issued							2,297,034