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Stockholders' Equity - Additional Information (Details) |  |  |  |  | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Jul. 14, 2017 USD ($) | Jul. 06, 2017 USD ($) | Jun. 16, 2017 $ / shares shares | Jun. 14, 2017 shares | Jun. 30, 2017 $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | Jan. 30, 2017 shares
Class Of Stock [Line Items] |  |  |  |  |  |  |  |
Dividends declared and paid |  |  |  |  |  | $ 11,277,000 |  |
Common stock, par value | $ / shares |  |  |  |  | $ 0.01 | $ 0.01 |  |
Proceeds from issuance of common stock |  | $ 82,700,000 |  |  |  | $ 76,829,000 | $ 49,592,000 |
Merger transaction, the number of shares into which each share of common stock issued and outstanding prior to merger was converted |  |  |  | 0.20 |  |  |  |
Merger transaction, number of fractional shares issued | shares |  |  |  | 0 |  |  |  |
Repurchase of shares |  |  |  |  |  | $ 15,003,000 |  |
Common Stock |  |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |  |
Issuance of common stock, net of issuance cost, shares | shares |  |  |  |  | 4,630,194 | 4,630,194 | 3,078,075 |
Issuance of common stock, price per share | $ / shares |  |  | $ 19.00 |  |  |  |  |
Under Writer | Common Stock |  |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |  |
Issuance of common stock, net of issuance cost, shares | shares |  |  |  |  | 855,000 |  |  |
Issuance of common stock, price per share | $ / shares |  |  |  |  | $ 19.00 |  |  |
Series B Preferred Stock |  |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |  |
Preferred stock, dividend rate, percentage |  |  |  |  |  |  | 7.50% |
Preferred stock, liquidation preference per share | $ / shares |  |  |  |  |  |  | $ 1,000 |
Preferred stock, shares issued | shares |  |  |  |  |  |  |  | 1,050
Dividends declared and paid |  |  |  |  |  | $ 776,000 |  |
Series A Preferred Stock |  |  |  |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |  |  |  |
Preferred stock, liquidation preference per share | $ / shares |  |  | $ 1,000 |  |  |  |  |
Shares of common stock used to determine cash value of repurchase | shares |  |  | 89.469 |  |  |  |  |
Repurchase of shares | $ 25,500,000 |  |  |  |  |  |  |
Dividends Payable |  |  |  |  |  | $ 10,500,000 |  |

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