Notes Payable - Additional Information (Detail) - USD ($)			1 Months Ended		12 Months Ended
	Mar. 24, 2017	Oct. 28, 2016	Mar. 31, 2017	Oct. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Loan borrowed amount					$ 500,000	$ 7,500,000	$ 12,500,000
Costs incurred in connection with loan agreement							673,000
Allocated fair value of warrants issued							923,000
Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Effective interest rate on note payable					17.70%
Loan and Security Agreement [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate floor				0.50%
Loan and Security Agreement [Member] | Horizon Technology Finance Corporation and Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Loan borrowed amount				$ 20,000,000
Debt instrument, interest rate floor				10.50%
Variable rate basis					one-month LIBOR
Interest rate description					Each loan tranche bears interest at a floating rate equal to 10.5% plus the amount by which the one-month LIBOR exceeds 0.5%.
Warrant exercise price condition					Exercise price equal to the lower of (a) the volume weighted average price per share of the Company’s Class A Common Stock, as reported on the principal stock exchange on which the Company’s Class A Common Stock is listed, for 10 trading days prior to the issuance of the applicable Warrants or (b) the closing price of a share of the Company’s Class A Common Stock on the trading day prior to the issuance of the applicable Warrants.
Warrants expiration period		7 years
Minimum cash balance required in deposit account				$ 2,500,000
Additional default interest rate				5.00%
Costs incurred in connection with loan agreement							700,000
Allocated fair value of warrants issued							900,000
Interest expense related to loan agreement					$ 3,100,000	$ 3,100,000	$ 400,000
Loan and Security Agreement [Member] | Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Prepayments for First 18 Months [Member]
Debt Instrument [Line Items]
Prepayment penalty fee				4.00%
Loan and Security Agreement [Member] | Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Prepayments Thereafter [Member]
Debt Instrument [Line Items]
Prepayment penalty fee				2.00%
Loan and Security Agreement [Member] | Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Tranche One [Member]
Debt Instrument [Line Items]
Loan borrowed amount				$ 12,500,000
Debt instrument, payment terms					The Company borrowed the first tranche of $12.5 million upon close of the Loan Agreement in October 2016. The first tranche required only monthly interest payments until May 1, 2018 followed by equal monthly payments of principal plus accrued interest through the scheduled maturity date on May 1, 2020.
Debt instrument, final payment				$ 800,000
Debt instrument, maturity date				May 01, 2020
Debt instrument, frequency of periodic Payment					monthly
Warrants to purchase shares of common stock		152,580
Exercise price of warrants		$ 6.39
Warrant shares percentage issued of loan amount		6.00%
Final payment, accrued additional interest expense					$ 800,000
Loan and Security Agreement [Member] | Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Tranche Two [Member]
Debt Instrument [Line Items]
Loan borrowed amount			$ 7,500,000
Debt instrument, payment terms					The Company borrowed the second tranche of $7.5 million in March 2017. The second tranche requires only monthly interest payments until October 1, 2018, followed by equal monthly payments of principal plus accrued interest through the scheduled maturity date on October 1, 2020.
Debt instrument, final payment				$ 500,000
Debt instrument, maturity date				Oct. 01, 2020
Debt instrument, frequency of periodic Payment					monthly
Warrants to purchase shares of common stock	38,006
Exercise price of warrants	$ 5.92
Warrant shares percentage issued of loan amount	3.00%	3.00%
Final payment, accrued additional interest expense					$ 500,000
Loan and Security Agreement [Member] | Horizon Technology Finance Corporation and Silicon Valley Bank [Member] | Tranche Three [Member]
Debt Instrument [Line Items]
Loan amount available for borrowing					$ 5,000,000
Tranches expiration date					Jun. 30, 2017