The Company and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Millions						12 Months Ended
	Feb. 21, 2017 shares	Feb. 14, 2017 USD ($) $ / shares shares	Jul. 21, 2015 USD ($) $ / shares shares	Jul. 15, 2015 USD ($) $ / shares shares	Jun. 29, 2015	Dec. 31, 2017 $ / shares shares	Dec. 31, 2015 shares	Dec. 31, 2016 $ / shares shares
Class of Stock [Line Items]
Shares issued, price per share | $ / shares			$ 17.00
Convertible preferred stock, conversion basis						On July 21, 2015, immediately prior to the closing of the IPO, all outstanding shares of convertible and redeemable convertible preferred stock converted into 18,361,953 shares of common stock, including an aggregate of 390,680 shares of common stock that were issued pursuant to the net exercise of 493,648 preferred stock warrants at the IPO price of $17.00 per share and an aggregate of 481,671 shares of common stock that were issued pursuant to the cash exercise of 481,671 preferred stock warrants. The IPO closed on July 21, 2015.
Number of common stock shares issued upon exercise of preferred stock warrants			390,680				481,671
Preferred stock warrants exercised			493,648
Capital stock, authorized			510,000,000
Common stock, shares authorized			500,000,000			500,000,000		500,000,000
Preferred stock, shares authorized			10,000,000			10,000,000		10,000,000
Preferred stock, par value | $ / shares			$ 0.001			$ 0.001		$ 0.001
Common stock, par value | $ / shares			$ 0.001			$ 0.001		$ 0.001
Reverse stock split, description						7.45-to-1
Reverse Stock Split, Conversion Ratio					0.13422818
Reverse stock split, effective date						Jul. 02, 2015
Series B and B-1 Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Cumulative dividends | $			$ 5.5
Series C and D Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Issuable common stock to redeemable convertible preferred stockholders			750,946				750,946
Common Stock [Member]
Class of Stock [Line Items]
Stock issued, shares						21,847,636	9,315,000
Redeemable convertible preferred stock converted into common stock			18,361,953
Common Stock [Member] | Series C and D Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Issuable common stock to redeemable convertible preferred stockholders							750,946
Cash Exercise [Member]
Class of Stock [Line Items]
Number of common stock shares issued upon exercise of preferred stock warrants			481,671
Preferred stock warrants exercised			481,671
Initial Public Offering [Member]
Class of Stock [Line Items]
Stock issued, shares				9,315,000
Shares issued, price per share | $ / shares				$ 17.00
Proceeds from issuance of common stock, net of deferred financing costs | $				$ 143.6
Follow on Offering [Member]
Class of Stock [Line Items]
Stock issued, shares	2,347,636	19,500,000
Shares issued, price per share | $ / shares		$ 1.35
Proceeds from issuance of common stock, net of deferred financing costs | $		$ 27.4
Underwriting discounts and commissions and offering expenses | $		$ 2.1