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Stockholders' and Mezzanine Equity - Additional Information (Detail) - USD ($) | 1 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Feb. 29, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Dec. 31, 2015
Series D Cumulative Redeemable Preferred Stock [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, dividend rate percentage |  | 7.00% |  |
Redeemable preferred stock, par value (in dollars per share) |  | $ 0.001 |  |
Sale of stock, consideration received on transaction |  | $ 55,100,000 |  |
Payments for repurchase of redeemable preferred stock |  | $ 38,500,000 |  |
Temporary equity, contract terms, change in control threshold |  | 50.00% |  |
Temporary equity, contract terms, minimum vote needed to trigger change in control from tender offer |  | 90.00% |  |
Preferred Stock [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, shares issued (in shares) |  | 2,264,000 |  | 2,150,000
Preferred Stock [Member] | Cantor Fitzgerald & Co [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Maximum remaining capacity to sell preferred stock under open market sale agreement |  | $ 37,200,000 |  |
Preferred Stock [Member] | Cantor Fitzgerald & Co [Member] | Series A Cumulative Redeemable Preferred Stock [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Redeemable preferred stock, dividend rate percentage | 7.75% |  |  |
Preferred Stock [Member] | Cantor Fitzgerald & Co [Member] | Series B Cumulative Redeemable Preferred Stock [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Maximum aggregate sales price of shares to be issued under open market sale agreement | $ 40,000,000 |  |  |
Redeemable preferred stock, dividend rate percentage | 7.50% |  |  |
Redeemable preferred stock, shares issued (in shares) |  | 114,000 |  |
Preferred stock, net proceeds from offering |  | $ 2,800,000 |  |
Preferred Stock [Member] | Cantor Fitzgerald & Co [Member] | Series D Cumulative Redeemable Preferred Stock [Member] |  |  |  |
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 |  | $ 37,300,000 |  |
Common Stock [Member] | Cantor Fitzgerald & Co [Member] |  |  |  |
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 |  | 65,000 |  |
Net proceeds under ATM program |  | $ 900,000 |  |
Maximum remaining capacity to sell preferred stock under open market sale agreement |  | $ 141,100,000 |  |
Redeemable preferred stock, shares issued (in shares) |  | 1,100,000 |  |
Preferred stock, net proceeds from offering |  | $ 17,700,000 |  |
Series D Preferred Stock ATM Program [Member] | Series D Cumulative Redeemable Preferred Stock [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Sale of stock, number of shares issued in transaction |  | 502,000 |  |
Sale of stock, consideration received on transaction |  | $ 12,500,000 |  |
Private Placement [Member] | Series D Cumulative Redeemable Preferred Stock [Member] |  |  |  |
Class of Stock [Line Items] |  |  |  |
Sale of stock, number of shares issued in transaction |  | 2,273,725 |  |
Redeemable preferred stock, liquidation preference (in dollars per share) |  | $ 25.00 |  |

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