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Stockholders' and Mezzanine Equity - Additional Information (Detail) - USD ($) |  | 1 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Apr. 11, 2018 | Feb. 29, 2016 | Sep. 30, 2018 | Dec. 31, 2017
Class of Stock [Line Items] |  |  |  |
Shares reclassified to common stock | 3,500,000 |  |  |
Senior common stock, authorized but unissued | 57,969 |  |  |
Capital stock, shares authorized | 100,000,000 |  |  |
Common stock, shares authorized | 87,700,000 |  | 87,700,000 | 34,200,000
Series A and B Preferred Stock ATM Programs |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, shares issued (in shares) |  |  | 2,264,000 | 2,264,000
Series D Cumulative Redeemable Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, dividend rate percentage (as percent) |  |  | 7.00% |
Temporary equity, contract terms, minimum vote needed to trigger change in control from tender offer (as percent) |  |  | 90.00% |
Cantor Fitzgerald & Co | Common Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, shares issued (in shares) |  |  | 500,000 |
Net proceeds from offering |  |  | $ 10,100,000 |
Maximum remaining capacity to sell preferred stock under open market sale agreement |  |  | $ 76,000,000 |
Cantor Fitzgerald & Co | Preferred Stock | Series A and B Preferred Stock ATM Programs |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, shares issued (in shares) |  |  | 0 |
Maximum remaining capacity to sell preferred stock under open market sale agreement |  |  | $ 37,200,000 |
Cantor Fitzgerald & Co | Preferred Stock | Series A Cumulative Redeemable Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, dividend rate percentage (as percent) |  | 7.75% |  |
Cantor Fitzgerald & Co | Preferred Stock | Series B Cumulative Redeemable Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, dividend rate percentage (as percent) |  | 7.50% |  |
Maximum aggregate sales price of shares to be issued under open market sale agreement |  | $ 40,000,000.0 |  |
Cantor Fitzgerald & Co | Preferred Stock | Series D Cumulative Redeemable Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Maximum remaining capacity to sell preferred stock under open market sale agreement |  |  | 18,600,000 |
Maximum aggregate sales price of shares to be issued under open market sale agreement |  |  | $ 50,000,000.0 |
Cantor Fitzgerald & Co | Series D Preferred Stock ATM Program | Series D Cumulative Redeemable Preferred Stock |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of shares issued (in shares) |  |  | 100,000 |
Consideration received |  |  | $ 2,200,000 |

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