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Stockholders' Equity and Mezzanine Equity - Additional Information (Detail) - USD ($) |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Mar. 28, 2015 | Jul. 31, 2017 | Feb. 29, 2016 | Mar. 31, 2011 | Mar. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2016
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Common stock, shares issued (in shares) |  |  |  |  |  | 28,384,016 | 24,882,758 |  |
Common stock, par value (in dollars per share) |  |  |  |  |  | $ 0.001 | $ 0.001 |  |
Shares intended to be offered pursuant to primary offering (in shares) |  |  |  | 3,000,000 |  |  |  |  |
Shares intended to be offered pursuant to senior common distribution reinvestment plan (in shares) |  |  |  | 500,000 |  |  |  |  |
Shares of senior common stock sold in offering (in shares) | 927,994 |  |  |  | 189,052 |  |  |  |
Senior common stock offering, gross proceeds |  |  |  |  |  | $ 86,260,000 | $ 118,362,000 | $ 50,165,000 |
Series D Cumulative Redeemable Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Minimum vote need to trigger change in control from tender offer (as percent) |  |  |  |  |  | 90.00% |  |  |
Senior Common Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Common stock, shares issued (in shares) |  |  |  | 3,500,000 |  |  |  |  |
Common stock, par value (in dollars per share) |  |  |  | $ 0.001 |  |  |  |  |
Common stock shares issued, selling price (in dollars per share) |  |  |  | $ 15.00 | $ 15.00 |  |  |  |
Shares intended to be offered pursuant to senior common distribution reinvestment plan (in shares) | 27,038 |  |  |  | 5,134 |  |  |  |
Net proceeds after deducting underwriting discount and commission |  |  |  |  | $ 2,600,000 |  |  |  |
Senior common stock offering, gross proceeds | $ 13,900,000 |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Offering price (in dollars per share) |  | $ 20.52 |  |  |  |  |  |  |
Overallotment sale of stock, price per share |  | $ 3,400,000 |  |  |  |  |  |  |
Senior common stock offering, gross proceeds |  | $ 22,500,000 |  |  |  |  |  |  |
Common Stock | Cantor Fitzgerald & Co |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Maximum aggregate sales price of shares to be issued under open market sale agreement |  |  | $ 160,000,000 |  |  |  |  |  |
Number of shares sold under open market sale agreement (in shares) |  |  |  |  |  | 2,133,000 |  |  |
Preferred Stock | Series D Cumulative Redeemable Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preferred stock, dividend rate (as percent) |  |  | 7.00% |  |  |  |  |  |
Preferred Stock | Cantor Fitzgerald & Co |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Remaining capacity to sell Preferred stock |  |  |  |  |  | $ 37,200,000 |  |  |
Preferred Stock | Cantor Fitzgerald & Co | Series A Cumulative Redeemable Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Preferred stock, dividend rate (as percent) |  |  | 7.75% |  |  |  |  |  |
Preferred Stock | Cantor Fitzgerald & Co | Series B Cumulative Redeemable Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Net proceeds from offering |  |  | $ 40,000,000 |  |  |  |  |  |
Preferred stock, dividend rate (as percent) |  |  | 7.50% |  |  |  |  |  |
Preferred Stock | Cantor Fitzgerald & Co | Series D Cumulative Redeemable Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Maximum aggregate sales price of shares to be issued under open market sale agreement |  |  |  |  |  |  |  |  | $ 50,000,000.0
Registered Public Offering | Common Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Number of shares issued (in shares) |  | 1,200,000 |  |  |  |  |  |  |
Sale of stock, number of shares issued in overallotment |  | 200,000 |  |  |  |  |  |  |
Series D Preferred Stock ATM Program | Cantor Fitzgerald & Co | Series D Cumulative Redeemable Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Number of shares issued (in shares) |  |  |  |  |  | 500,000 |  |  |
Net proceeds from sale of shares |  |  |  |  |  | $ 12,700,000 |  |  |
Cantor Fitzgerald & Co | Common Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Net proceeds from offering |  |  |  |  |  | 45,500,000 |  |  |
Remaining capacity to sell Preferred stock |  |  |  |  |  | 86,300,000 |  |  |
Cantor Fitzgerald & Co | Preferred Stock | Series D Cumulative Redeemable Preferred Stock |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |
Remaining capacity to sell Preferred stock |  |  |  |  |  | $ 20,800,000 |  |  |

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