Stockholders' Deficit - Additional Information (Detail)								12 Months Ended
	Jan. 07, 2018 USD ($) $ / shares shares	Sep. 20, 2017 USD ($) $ / shares	May 03, 2017	Mar. 28, 2017 USD ($) $ / shares shares	Mar. 15, 2017	Feb. 14, 2017 USD ($) $ / one_hundred_dollars_of_preferred_stock shares	May 03, 2016 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares $ / one_hundred_dollars_of_preferred_stock shares	Dec. 31, 2016 USD ($) $ / shares shares	Mar. 08, 2017 shares	Mar. 07, 2017 shares	Dec. 31, 2011 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized | shares								300,000,000	300,000,000
Common stock, par value (usd per share) | $ / shares								$ 0.001	$ 0.001
Preferred stock, shares authorized | shares								10,000,000		50,000,000	10,000,000
Reverse stock split, shares received for each common share					0.125
Net proceeds on issuance stock								$ 48,792,000	$ 23,964,000
Average Market capitalization						$ 300,000,000
Stock-based compensation expense related to stock options								5,600,000	3,800,000
Percent of capital stock authorized issuance for ESPP			2.00%
Purchase price of common stock (not less than) (percent)			85.00%
Compensation expense								5,583,000	3,792,000
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense								$ 100,000
ESPP shares granted, shares | shares								0
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share price (usd per share) | $ / shares		$ 1.00
2008 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options									0
Compensation expense								$ 0	$ 200,000
Units cancelled, shares | shares									20,307,149
2014 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options								700,000
Compensation expense								$ 0	$ 700,000
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Initial shares available, shares | shares								2,116,004
Available for issuance, shares | shares								405,620
Contractual life								10 years
Share based compensation related to options issued								$ 3,900,000	$ 1,000,000
Weighted average grant date fair value of options granted (usd per share) | $ / shares								$ 4.39	$ 24.56
Options exercised | shares								0
Unrecognized share-based compensation expense related to unvested stock options								$ 7,000,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition								2 years 1 month 5 days
2016 Plan | Time-Based Restricted Stock Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense related to stock options								$ 1,700,000	$ 600,000
Unrecognized share-based compensation expense related to unvested stock options								0
Fair value of restricted stock on date of issuance (usd per share)								$ 100,000	2,300,000
2016 Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period								3 years
2016 Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period								4 years
Total grant date fair value of options vested (less than)								$ 4,709,000	$ 100,000
Aspire Capital | Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum shares available for purchase, shares | shares	1,375,868
Shares limited to purchase, percent	19.99%
Aspire Capital | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares limited to purchase, percent	19.99%
Aspire Capital | Subsequent Event | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share price (usd per share) | $ / shares	$ 1.0
Aspire Capital | Purchase Agreement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock purchase agreement (up to)		$ 20,000,000
Term of purchase agreement		30 months
Aspire Capital | Purchase Agreement | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock purchase agreement (up to)	$ 20,000,000
Term of purchase agreement	30 months
Commitment to purchase shares, shares | shares	50,000
Series A preferred stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of preferred stock converted to common, shares | shares						1
Cumulative annual dividends per $100 | $ / one_hundred_dollars_of_preferred_stock						8		8
Term loan | CRG conversion
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Principal balance converted				$ 25,000,000				$ 25,000,000
Term loan | WCAS conversion
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Principal balance converted								$ 2,500,000
Term loan | Debt conversion CRG and WCAS
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Principal balance converted						$ 27,500,000
Restructured debt | Restructured WCAS Note | WCAS Capital Partners IV, L.P.
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Debt, face amount				$ 2,500,000								$ 5,000,000
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock, shares | shares								5,250,000
Proceeds from private placement, net							$ 24,000,000
Common Stock | Existing investor
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from private placement							$ 20,000,000
Common Stock | Private placement offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock, shares | shares							600,000		634,858
Share price (usd per share) | $ / shares							$ 40
Common Stock | Public Offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Issuance of common stock, shares | shares				5,250,000
Share price (usd per share) | $ / shares				$ 10
Net proceeds on issuance stock				$ 48,800,000
Common Stock | Public Offering | Existing investor
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net proceeds on issuance stock				$ 40,000,000
Preferred Stock | CRG conversion | Series A preferred stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued upon conversion of debt, shares | shares				2,500,000				2,500,000
Preferred Stock | WCAS conversion | Series A preferred stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares issued upon conversion of debt, shares | shares				250,000				250,000