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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 | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | [1] | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Earnings Per Share Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Net income | $ (5,206) | $ 2,104 | $ 8,525 | $ 5,734 | $ (1,926) | $ 4,789 | $ 4,830 | $ 2,944 | $ 11,157 | [2] | $ 10,637 | [2] | $ 4,763
Weighted average number of shares - basic | 42,216,987 | 41,869,528 | 41,567,848 | 41,507,941 | 36,527,250 | 33,873,479 | 33,128,518 | 32,888,659 | 41,764,230 | [2] | 34,104,476 | [2] | 32,546,840
Weighted average number of shares outstanding - diluted | 42,216,987 | 43,463,982 | 43,550,226 | 43,503,889 | 36,527,250 | 35,297,585 | 34,270,337 | 33,944,876 | 42,766,843 | [2] | 35,416,910 | [2] | 33,936,099
Warrants |  |  |  |  |  |  |  |  |  |  |  |  |
Earnings Per Share Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Shares underlying |  |  |  |  |  |  |  |  | 0 |  | 0 |  | 300,846
Options |  |  |  |  |  |  |  |  |  |  |  |  |
Earnings Per Share Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Shares underlying |  |  |  |  |  |  |  |  | 1,002,613 |  | 1,312,434 |  | 1,088,413
[1] | As discussed previously, the fourth quarter 2017 results include the results of ARI subsequent to the acquisition on October 10, 2017.
[2] | The sum of the net earnings per share may not add up to the full year amount due to rounding and because the quarterly calculations are based on varying numbers of shares outstanding.

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