Convertible Preferred Stock - Additional Information (Detail) - USD ($)			1 Months Ended						3 Months Ended	12 Months Ended
	Feb. 11, 2016	Dec. 17, 2015	Dec. 31, 2017	Sep. 30, 2016	Sep. 30, 2015	May 31, 2015	Sep. 30, 2014	Jan. 31, 2014	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 30, 2014
Class of Stock [Line Items]
Preferred Shares Authorized												110,057,398
Preferred stock, par value			$ 0.001							$ 0.001	$ 0.001	$ 0.001
Stock issued during the period, share			0							0	0
Issuance cost	$ 4,000,000			$ 800,000
Preferred dividend description										In September 2015, in connection with the Series B preferred stock financing, (1) the right of the holders of Series A preferred stock to receive accruing dividends, whether or not declared, at a rate of 8.0% per year of the Original Issue Price per share (as described below) ceased as of August 31, 2015, at which time such cumulative accruing dividends totaled $36.0 million, and (2) the right of holders of Series A preferred stock to receive a cash payment of accruing dividends upon the automatic conversion of the Series A preferred stock into common stock was modified such that all previously accrued but unpaid dividends on Series A preferred stock will be paid in shares of common stock, at a price of $13.9019172 per share, upon such conversion of the Series A preferred stock. If as of January 1, 2016, the Series A preferred stock has not automatically converted into shares of common stock according to its terms, the holders of the Series A preferred stock will be entitled to receive dividends at the rate of 8.0% per year of the respective Original Issue Price per share, commencing as of the respective original issuance date of the Series A preferred stock. These amendments to the Series A preferred stock dividend rights were accounted for as a modification of preferred stock based on a quantitative assessment of the change in the fair value that resulted from the modification of the dividend rights on the modification date. The decrease in the fair value of Series A preferred stock measured immediately before the modification and immediately after the modification, equal to $0.2 million, was recorded as a deemed dividend from holders of preferred stock to the holders of common stock, resulting in an increase to additional paid-in capital of $0.2 million and a corresponding decrease of $0.2 to the carrying value of the Series A preferred stock.
Modifications of preferred stock									$ 1,378,000		$ 1,378,000	$ 9,724,000
Proceeds of IPO qualified for conversion of preferred stock											$ 46,500,000
IPO [Member]
Class of Stock [Line Items]
Preferred stock, par value	$ 0.001		$ 0.001							$ 0.001
Stock issued during the period, share			0							0	0
Preferred stock, issuance price	$ 8.00
Stock issued during the period, share	6,250,000
Number shares issued as payment for accrued dividend	2,590,742
Proceeds of IPO qualified for conversion of preferred stock	$ 42,500,000
Number shares issued in share conversion	9,699,600
Maximum [Member]
Class of Stock [Line Items]
Issuance cost			$ 400,000
Minimum [Member]
Class of Stock [Line Items]
Proceeds of IPO qualified for conversion of preferred stock										$ 50,000,000
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Conversion of convertible preferred stock, Shares											(104,854,769)
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Shares Authorized												76,000,000
Stock issued during the period, share								5,000,000
Preferred stock, issuance price								$ 1.00
Proceeds from issuance of preferred stock value								$ 4,900,000
Issuance cost								$ 100,000
Conversion price per share						$ 10.8102	$ 3.6030397						$ 10.8102
Modifications of preferred stock												$ 11,481,000
Preferred stock percentage		8.00%			8.00%					8.00%
Cumulative accrued dividends	$ 36,000,000				$ 36,000,000
Dividends per share					$ 13.9019172
Series A Preferred Stock [Member] | Dividend [Member]
Class of Stock [Line Items]
Modifications of preferred stock						$ 10,600,000	$ 6,000,000
Series A Preferred Stock [Member] | Additional Paid-in Capital [Member]
Class of Stock [Line Items]
Modifications of preferred stock						10,600,000	1,000,000
Modifications of preferred stock		$ (800,000)										11,481,000
Modifications of preferred stock					$ 200,000
Series A Preferred Stock [Member] | Accumulated Deficit [Member]
Class of Stock [Line Items]
Modifications of preferred stock							$ 5,000,000
Series A Preferred Stock [Member] | Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Modifications of preferred stock		800,000				$ 10,600,000						$ (11,481,000)
Modifications of preferred stock					$ 200,000
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Shares Authorized												34,057,398
Preferred stock, issuance price					$ 1.286
Proceeds from issuance of preferred stock value					$ 21,100,000
Issuance cost					$ 900,000							$ 910,000
Modifications of preferred stock												(26,000)
Stock issued during the period, share					17,107,303
Aggregate promissory notes converted					$ 15,000,000
Aggregate accrued interest					$ 800,000
Conversion of convertible preferred stock, Shares					12,284,466
Share price					$ 1.286
Preferred stock percentage					8.00%
Series B Preferred Stock [Member] | Additional Paid-in Capital [Member]
Class of Stock [Line Items]
Modifications of preferred stock												(26,000)
Series B Preferred Stock [Member] | Additional Paid-in Capital [Member] | Maximum [Member]
Class of Stock [Line Items]
Modifications of preferred stock		(100,000)
Series B Preferred Stock [Member] | Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Modifications of preferred stock												$ 26,000
Stock issued during the period, share												17,107,303
Conversion of convertible preferred stock, Shares												12,284,466
Series B Preferred Stock [Member] | Convertible Preferred Stock [Member] | Maximum [Member]
Class of Stock [Line Items]
Modifications of preferred stock		$ 100,000