Debt Facility - Additional Information (Detail) $ / shares in Units, $ in Thousands		9 Months Ended		12 Months Ended
	Apr. 28, 2017 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Tranche
Line Of Credit Facility [Line Items]
Number of tranches | Tranche				2
Repayment of principal debt and fees			$ 3,567
Final payment due on date of maturity		$ 900
Warrants issued to purchase shares of common stock | shares		1,665,170
Amortization of debt discount		$ 400
April 2017 Warrants [Member]
Line Of Credit Facility [Line Items]
Warrants, expected term		10 years
April 2017 Warrants [Member] | Volatility [Member]
Line Of Credit Facility [Line Items]
Warrants, expected rate		71.42
April 2017 Warrants [Member] | Risk Free Interest Rate [Member]
Line Of Credit Facility [Line Items]
Warrants, expected rate		2.33
April 2017 Warrants [Member] | Dividend Yield [Member]
Line Of Credit Facility [Line Items]
Warrants, expected rate		0
June 2017 Warrants [Member]
Line Of Credit Facility [Line Items]
Warrants, expected term		10 years
June 2017 Warrants [Member] | Volatility [Member]
Line Of Credit Facility [Line Items]
Warrants, expected rate		71.57
June 2017 Warrants [Member] | Risk Free Interest Rate [Member]
Line Of Credit Facility [Line Items]
Warrants, expected rate		2.16
June 2017 Warrants [Member] | Dividend Yield [Member]
Line Of Credit Facility [Line Items]
Warrants, expected rate		0
Common Stock [Member] | April 2017 Warrants [Member]
Line Of Credit Facility [Line Items]
Common stock warrants issued to purchase common stock percentage on funded amount	3.00%
Warrants issued to purchase shares of common stock | shares	24,725
Exercise price of warrants per share | $ / shares	$ 9.10
Warrants expiration term	10 years
Warrants expiration date	Apr. 28, 2027
Common Stock [Member] | June 2017 Warrants [Member]
Line Of Credit Facility [Line Items]
Common stock warrants issued to purchase common stock percentage on funded amount	3.00%
Exercise price of warrants per share | $ / shares	$ 5.39
Warrants expiration term	10 years
Warrants expiration date	Jun. 15, 2027
Additional warrants issued to purchase shares of common stock | shares	41,736
Loan and Security Agreement [Member]
Line Of Credit Facility [Line Items]
Repayment of principal debt and fees				$ 3,700
Loan and Security Agreement [Member] | Silicon Valley Bank [Member] | Term Loan [Member]
Line Of Credit Facility [Line Items]
Loan and security agreement, maximum amount				$ 15,000
Interest rate, basis spread	4.25%
Loan agreement payment terms		The Loan Agreement bears interest at the prime rate reported in The Wall Street Journal, plus a spread of 4.25%. Interest only payments are due through September 2018 followed by monthly payments of principal plus interest over the following 30 months. Since the second tranche was fully extended, the interest only period was extended for an additional 6 months, through March 2019 followed by monthly payments of principal plus interest over the following 24 months through the maturity date of March 1, 2021 under the Loan Agreement provisions.
Debt instrument interest only payment maturity date	2018-09
Debt instrument principal and interest payment period	30 months
Debt instrument extended interest only payment maturity date	2019-03
Debt instrument interest only payment extended period	6 months
Debt instrument principal and interest payment extended period	24 months
Debt instrument, frequency of periodic interest payments		monthly
Debt instrument maturity date	Mar. 01, 2021
Customary closing fees	6.00%
Payments of debt issuance costs	$ 100