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

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for option under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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Number of additional shares authorized for issuance under share-based payment arrangement.

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

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Percentage of vesting of award under share-based payment arrangement.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Stock-Based Compensation - Additional Information (Detail) - USD ($) shares in Thousands, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	May 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense associated					$ 181
Intrinsic value of options outstanding		$ 10,501		$ 10,501
Intrinsic value of options vested		1,764		1,764
Intrinsic value of options unvested		8,737		8,737
Total compensation cost related to stock options not yet recognized		12,719		$ 12,719
Total compensation cost related to stock options not yet recognized, Weighted average period for recognition				2 years 9 months 18 days
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense associated		1,688	$ 1,506	$ 4,655	7,281
Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense associated		$ 0	$ 89	$ 154	$ 161
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares of common stock authorized	8,200
Shares of common stock available for grant		8,535		8,535
2015 Plan \| Quarterly thereafter for the following three years
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options awards, vesting percentage				6.25%
Options vested period				3 years
2015 Plan \| On first anniversary of the date of grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options awards, vesting percentage				25.00%
2015 Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option awards, expiration period				10 years