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Common Stock - Additional Information (Detail) |  |  |  |  | 12 Months Ended
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 $ / shares shares | Feb. 01, 2017 USD ($) $ / shares shares | Nov. 29, 2016 USD ($) $ / shares shares | Nov. 09, 2016 USD ($) $ / shares shares | Dec. 31, 2017 $ / shares shares | Dec. 31, 2016 shares
Class Of Stock [Line Items] |  |  |  |  |  |
Common stock split ratio |  |  |  | 2,200 |  |
Common stock, shares authorized | 350,000,000 |  |  | 350,000,000 | 350,000,000 | 350,000,000
Preferred stock, undesignated shares authorized |  |  |  | 10,000,000 |  |
Common Stock |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |
Sale of shares of common stock |  | 1,500,000 |  |  | 1,500,000 | 12,577,500
Purchase price per share | $ / shares | $ 17.50 | $ 17.50 |  |  | $ 17.50 |
Proceeds from issuance of common stock | $ |  |  | $ 0 |  |  |
Proceeds from issuance of common stock net of underwriting discounts and expenses | $ |  | $ 24,200,000 |  |  |  |
Stock sold by selling shareholders | 4,450,000 |  |  |  |  |
Underwriters option period | 30 days |  |  |  |  |
Common Stock | Maximum |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |
Additional common stock shares granted by selling shareholders to underwriters option | 893 |  |  |  | 893 |
Common Stock | Initial Public Offering |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |
Sale of shares of common stock |  |  |  | 11,700,000 |  |
Purchase price per share | $ / shares |  |  |  | $ 11.00 |  |
Net proceed received from IPO after deducting underwriting discounts and commissions | $ |  |  |  | $ 121,000,000 |  |
Common Stock | Over-Allotment Option |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |
Purchase price per share | $ / shares |  |  | $ 11.00 |  |  |
Additional common stock, shares issued to underwriters |  |  | 878,000 |  |  |
Net proceed received from underwriter option exercise after deducting underwriting discounts and commissions | $ |  |  | $ 9,700,000 |  |  |

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