Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Jul. 21, 2015	Jun. 11, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Preferred stock, shares authorized	10,000,000			10,000,000	10,000,000
Preferred stock, par value	$ 0.001			$ 0.001	$ 0.001
Preferred stock, shares outstanding				0	0
Number of shares issued upon exercise of stock warrants	390,680		481,671
Number of convertible preferred stock issued upon exercise of stock warrants			390,032
Percentage of accrued but unpaid dividends paid in shares		50.00%
Percentage of accrued but unpaid dividends forfeited		50.00%
Share price		$ 5.215
Cumulative unpaid dividends in arrears	$ 18.9
Series C Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Number of shares issued upon exercise of stock warrants	93,017		37,385
Number of convertible preferred stock issued upon exercise of stock warrants			93,017
Cumulative unpaid dividends in arrears	$ 1.7
Issuable common stock to redeemable convertible preferred stockholders			161,536
Fair value of common stock to redeemable convertible preferred stockholders			$ 4.4
Series D Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Cumulative unpaid dividends in arrears	$ 6.1
Issuable common stock to redeemable convertible preferred stockholders			589,410
Fair value of common stock to redeemable convertible preferred stockholders			$ 16.0
Series B-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Number of shares issued upon exercise of stock warrants	291,164		444,286
Number of convertible preferred stock issued upon exercise of stock warrants			291,165
Cumulative unpaid dividends in arrears	$ 1.7
Accruing dividends			$ 0.8
Series B Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Number of shares issued upon exercise of stock warrants	6,499
Number of convertible preferred stock issued upon exercise of stock warrants			5,850
Cumulative unpaid dividends in arrears	$ 9.4
Accruing dividends			$ 4.7
Series A Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock converted into common stock shares	252,817
Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Preferred stock converted into common stock shares	18,109,136		(17,913,767)
Series B and B-1 Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Accruing dividends			$ 5.5
Series C and D Redeemable Convertible Preferred Stock [Member]
Temporary Equity [Line Items]
Issuable common stock to redeemable convertible preferred stockholders	750,946		750,946
Fair value of common stock to redeemable convertible preferred stockholders			$ 20.4