Retirement Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Employers' sponsors contribution retirement plan	90.00%
Contribution to retirement plan, percentage of employee contribution	100.00%
Employers' sponsors contribution retirement plan amount	$ 0.5	$ 0.4	$ 0.2
Maximum [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of employee contribution to retirement plan	5.00%

Pension Plans, Postretirement and Other Employee Benefits [Line Items]

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Amount of discretionary contributions made by an employer to a defined contribution plan.

No definition available.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

No definition available.

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Percentage employer matches of the employee's percentage contribution matched.

No definition available.

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Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

No definition available.

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