Stockholders' Equity - 2018 Equity Financing - Additional Information (Detail) - USD ($)					1 Months Ended	3 Months Ended			9 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 25, 2018	Jun. 22, 2018	Mar. 28, 2018	Sep. 30, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Jun. 21, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common Stock, Shares Authorized		150,000,000									150,000,000	150,000,000
Change in fair value of derivative liability						$ (18,886,000)			$ (45,164,000)
Exercise price of Second Tranche Warrants					$ 1.23	$ 1.23			$ 1.23		$ 1.23	$ 1.23		$ 0
First Tranche Securities Purchase Agreement [Member]
Class of Stock [Line Items]
Common stock issued to investors				8,606,324
Common stock price per share				$ 1.10
Gross proceeds from issuance of common stock				$ 9,500,000
Portion of First Tranche net proceeds allocated to equity				$ 4,600,000
Second Securities Purchase Agreement [Member]
Class of Stock [Line Items]
Gross proceeds to be received from issuance of common shares, subject to stockholder approval			$ 25,500,000
2018 Equity Financing [Member]
Class of Stock [Line Items]
Common Stock, Shares Authorized													120,000,000
Share issuance costs								$ 343,000
2018 Equity Financing [Member] | Second Tranche Transaction [Member]
Class of Stock [Line Items]
Units issued to investors, with each Unit consisting of one common share and one warrant to purchase one common share		20,184,224
Unit price per share		$ 1.265
Gross proceeds from issuance of units		$ 25,500,000
Fair value of derivative liability								6,900,000
Change in fair value of derivative liability								$ 2,200,000		$ 22,200,000
Reclassification of the derivative liability balance to equity upon the exercise of the Second Tranche Warrants										$ 29,100,000
2018 Equity Financing [Member] | Second Tranche Warrants [Member]
Class of Stock [Line Items]
Change in fair value of derivative liability	$ 1,600,000				$ 18,900,000		$ 18,200,000
Reclassification of the derivative liability balance to equity upon the exercise of the Second Tranche Warrants					$ 38,700,000
Exercise price of Second Tranche Warrants					$ 1.43	$ 1.43			$ 1.43