Long-Term Debt - Schedule of Future Minimum Payments under Loan Agreement (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Carrying value of notes payable	$ 41,905	$ 31,777
Long-term debt, alternative
Carrying value of notes payable	41,905	31,777
Notes payable, gross	42,000	$ 32,000
Loan and Security Agreement [Member] \| Oxford and Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Three months ended December 31, 2018	955
2019	10,437
2020	15,446
2021	14,283
2022	12,684
Carrying value of notes payable	53,805
Long-term debt, alternative
Carrying value of notes payable	53,805
Less: Interest and final payment	(11,805)
Notes payable, gross	$ 42,000

Long term debt maturities repayments interest and final payment.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the remainder of the fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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