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Stockholders' and Mezzanine Equity - Additional Information (Detail) - USD ($) | 1 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Feb. 29, 2016 | Feb. 29, 2012 | Jun. 30, 2017 | Jan. 11, 2017 | Dec. 31, 2016 | Jun. 30, 2016
Class of Stock [Line Items] |  |  |  |  |  |
Partnership units authorized (in shares) |  |  |  | 0 |  |
Mandatorily redeemable preferred stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Redeemable preferred stock, shares issued (in shares) |  | 1,540,000 |  |  |  |
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 |
Redeemable preferred stock (in shares) |  |  | 5,350,000 |  | 5,350,000 |
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% |  |  |
Series D Cumulative Redeemable Preferred Stock | Series D Preferred Stock ATM Program |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Number of shares issued (in shares) |  |  | 300,000 |  |  |
Consideration received |  |  | $ 6,900,000 |  |  |
Series C Preferred Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Redeemable preferred stock, dividend rate percentage (as percent) |  | 7.125% |  |  |  |
Series C Preferred Stock | Mandatorily redeemable preferred stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Redeemable preferred stock (in shares) |  |  |  | 0 | 160,000 |
Cantor Fitzgerald & Co | Common Stock |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Maximum aggregate sales price of shares to be issued under open market sale agreement | $ 160,000,000.0 |  |  |  |  |
Redeemable preferred stock, shares issued (in shares) |  |  | 900,000 |  |  |
Net proceeds from offering |  |  | $ 17,900,000 |  |  |
Maximum remaining capacity to sell preferred stock under open market sale agreement |  |  | $ 114,300,000 |  |  |
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] |  |  |  |  |  |
Maximum aggregate sales price of shares to be issued under open market sale agreement | $ 40,000,000.0 |  |  |  |  |
Redeemable preferred stock, dividend rate percentage (as percent) | 7.50% |  |  |  |  |
Cantor Fitzgerald & Co | Preferred Stock | 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
Maximum remaining capacity to sell preferred stock under open market sale agreement |  |  | $ 26,700,000 |  |  |

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