Common Stock and Stock Warrants - Additional Information (Detail)			1 Months Ended			12 Months Ended
	Oct. 30, 2017 USD ($) $ / shares shares	Aug. 12, 2016 $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Aug. 31, 2016 USD ($)	Oct. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) vote $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	May 31, 2015 USD ($) shares
Class of Stock [Line Items]
Shares authorized at incorporation | shares						150,000,000
Common stock, shares authorized | shares						130,000,000	130,000,000
Common stock, par value (usd per share) | $ / shares						$ 0.0001	$ 0.0001
Preferred stock, shares authorized | shares						20,000,000	20,000,000
Preferred stock, par value (usd per share) | $ / shares						$ 0.0001	$ 0.0001
Common stock, number of votes per share | vote						1
Minimum percent of combined voting power						75.00%
Common stock, voting rights, majority of minority ownership percentage						50.00%
Common stock, voting rights, majority of minority ownership percentage, threshold for excluding percentage						15.00%
Maximum aggregate offering price of securities under shelf registration									$ 200,000,000
Maximum number of shares to be sold under shelf registration | shares									2,500,000.0
Maximum aggregate offering price under the shelf registration									$ 75,000,000.0
Value of stock issued during the period					$ 34,500,000	$ 37,940,000	$ 12,355,000	$ 32,150,000
Proceeds from issuance of common stock, net of issuance costs					32,100,000	37,998,000	12,355,000	32,189,000
Payments for commissions					2,100,000	0
Total costs					$ 280,000	19,000	13,000	283,000
Amount available for issuance						112,500,000
Common stock issued for services						256,000
Common stock issued for services						$ 256,000	$ 0	$ 0
Number of warrants outstanding (in shares) | shares							240,620
Exercise price (usd per share) | $ / shares						$ 92.99	$ 92.99
At-The-Market Sales Agreement
Class of Stock [Line Items]
Maximum aggregate offering price						$ 62,200,000
Value of stock issued during the period						600,000	$ 12,200,000
Proceeds from issuance of common stock, net of issuance costs						468,000	11,700,000
Payments for commissions							366,000
Total costs						$ 114,000	$ 173,000
Common stock sold (in shares) | shares						100,000	1,500,000
Additional Follow-On Offering
Class of Stock [Line Items]
Value of stock issued during the period			$ 40,300,000
Proceeds from issuance of common stock, net of issuance costs			37,500,000
Payments for commissions			2,400,000
Total costs			$ 362,000
Common stock sold (in shares) | shares			10,100,000
Common stock sold, price (usd per share) | $ / shares			$ 4.00
Private Placement
Class of Stock [Line Items]
Common stock sold (in shares) | shares		118,243
Common stock sold, price (usd per share) | $ / shares		$ 5.92
Proceeds from issuance of private placement				$ 700,000
Weighted Average | At-The-Market Sales Agreement
Class of Stock [Line Items]
Common stock sold, price (usd per share) | $ / shares						$ 6	$ 7.90
Restricted Stock
Class of Stock [Line Items]
Common stock issued for services (in shares) | shares	60,000
Percent of shares to be surrendered at termination	40.00%
Shares quoted at market price (in shares) | shares	36,000
Percent of shares quoted at market price	60.00%
Common stock issued for services	$ 207,000
Market price of shares (in usd per share) | $ / shares	$ 5.75
Shares adjusted to fair value (in shares) | shares	24,000
Expense recognition period	180 days
Warrants to purchase common stock
Class of Stock [Line Items]
Number of warrants outstanding (in shares) | shares						240,620