Debt - Additional Information (Detail) - Silicon Valley Bank and Oxford finance - Term Loan - USD ($)		12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Line of credit facility, effective date		Oct. 31, 2017
Line of credit facility, description		the lenders agreed to make term loans available to the Company in an aggregate amount of $100 million, consisting of (i) an aggregate amount of $50 million available on the Effective Date and (ii) an aggregate amount of $50 million available to be drawn at the Company’s option between March 31, 2018 and June 30, 2018, provided the Company has achieved a specified minimum revenue milestone and no event of default is occurring.
Term loan, maximum borrowing capacity	$ 100,000,000
Amount available on effective date under agreement	50,000,000
Line of credit facility available upon achievement of specified minimum revenue milestone	$ 50,000,000
Percentage of issued and outstanding capital stock of its subsidiary pledged	65.00%
Interest rate, description		The term loans under the credit facility bear interest at an annual rate equal to the greater of (i) 7.75% and (ii) the sum of (a) the “prime rate,” as reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue, plus (b) 3.5%. The Company is required to make monthly interest-only payments on each outstanding term loan commencing on the first calendar day of the calendar month following the funding date of such term loan, and continuing on the first calendar day of each calendar month thereafter through December 1, 2019.
Annual interest rate	7.75%
Redemption period start date	Dec. 01, 2019
Line of credit facility closing date	Oct. 31, 2022
Percentage of original principal amount payable in final payment	7.50%
Covenant, description		The credit facility includes affirmative and negative covenants applicable to the Company, its current subsidiary and any subsidiaries it may create in the future. The affirmative covenants include, among others, covenants requiring the Company to maintain its corporate existence and governmental approvals, deliver certain financial reports, maintain insurance coverage and satisfy certain requirements regarding deposit accounts. The Company must also achieve product revenue, measured as of the last day of each fiscal quarter on a trailing three-month basis, that is (i) greater than or equal to 70% of its revenue target set forth in its board-approved projections for the 2017 fiscal year, (ii) greater than or equal to 50% of its revenue target set forth in its board-approved projections for the 2018 fiscal year, and (iii) greater than or equal to 50% of its revenue target set forth in its board-approved projections for the 2019 fiscal year. New minimum revenue levels will be established for each subsequent fiscal year by mutual agreement of the Company, SVB, as administrative agent, and the lenders. The negative covenants include, among others, restrictions on the Company’s transferring collateral, incurring additional indebtedness, engaging in mergers or acquisitions, paying dividends or making other distributions, making investments, creating liens, selling assets and suffering a change in control, in each case subject to certain exceptions.
Credit facility covenants, percentage of target revenue to be achieved for 2017 fiscal year	70.00%
Credit facility covenants, percentage of target revenue to be achieved for 2018 fiscal year	50.00%
Credit facility covenants, percentage of target revenue to be achieved for 2019 fiscal year	50.00%
Percentage of additional interest rate to be charged on the event of default	5.00%
Amount of indebtedness or judgments against company to be considered as threshold limit for default	$ 500,000
First Anniversary of Funding
Debt Instrument [Line Items]
Prepayment fee, percentage	2.00%
Between First Anniversary and Second Anniversary
Debt Instrument [Line Items]
Prepayment fee, percentage	1.00%
Prime Rate
Debt Instrument [Line Items]
Basis spread on variable rate	3.50%