Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Options Issued and Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning, Shares Available for Grant	1,670,341
Shares reserved for issuance, Shares Available for Grant	641,239
Options granted, Shares Available for Grant	(871,010)
Outstanding, ending, Shares Available for Grant	1,413,610	1,670,341
Outstanding, beginning, Number of Options	195,668
Options granted, Number of Options	871,010
Outstanding, ending, Number of Options	1,066,678	195,668
Exercisable, Number of Options	25,487
Vested and expected to vest, Number of Options	1,066,678
Outstanding, beginning, Weighted-Average Exercise Price per Option	$ 4.49
Options granted, Weighted-Average Exercise Price per Option	13.09
Outstanding, ending, Weighted-Average Exercise Price per Option	11.52	$ 4.49
Exercisable, Weighted-Average Exercise Price per Option	0.79
Vested and expected to vest, Weighted-Average Exercise Price per Option	$ 11.52
Outstanding, beginning, Weighted-Average Remaining Contract Term	9 years 5 months 4 days	9 years 8 months 1 day
Exercisable, Weighted-Average Remaining Contract Term	8 years 8 months 12 days
Vested and expected to vest, Weighted-Average Remaining Contract Term	9 years 5 months 4 days
Outstanding, ending, Aggregate Intrinsic Value	$ 3,901
Exercisable, Aggregate Intrinsic Value	366
Vested and expected to vest, Aggregate Intrinsic Value	$ 3,901
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options granted, Shares Available for Grant	(2,000)
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options granted, Shares Available for Grant	(24,960)

Share based compensation arrangement by share based payment award number of shares available for grant, shares granted.

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Share based compensation arrangement by share based payment award number of shares available for grant shares granted, shares reserved for issuance.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Net number of non-option equity instruments granted to participants.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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