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401 (k) Plan - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract] |  |  |
Maximum contribution by employee, percentage of pretax earnings | 60.00% |  |
Percentage of company's matching contribution with respect to each participant's contribution | 100.00% |  |
Company matching contributions to maximum employees eligible compensation | 5.00% |  |
Company contributions to 401 (k) plan | $ 3.3 | $ 2.1 | $ 1.0

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