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Summary of Significant Accounting Policies - Reconciliation of Shares Used to Determine Net Income per Share (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Earnings Per Share Disclosure [Line Items] |  |  |  |  |
Net (loss) income | $ (13,880) | $ 2,104 | $ (47,552) | $ 16,362 | $ 11,157
Weighted average number of shares - basic | 42,530,476 | 41,869,528 | 42,445,926 | 41,648,439 |
Shares underlying options | 0 | 1,594,454 | 0 | 1,524,988 |
Weighted average number of shares outstanding – diluted | 42,530,476 | 43,463,982 | 42,445,926 | 43,173,427 |

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