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Stockholders' Equity and Mezzanine Equity - Summary of Changes in Stockholders' Equity (Detail) | 1 Months Ended | 12 Months Ended
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Feb. 29, 2012 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Series C Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) | 7.125% |  |  |
Ordinary Income | Common Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Common stock, dividend rate (as percent) |  | 39.63189% | 28.81758% | 18.80114%
Ordinary Income | Senior Common Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Common stock, dividend rate (as percent) |  | 100.00% | 100.00% | 91.71368%
Ordinary Income | Series A Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 100.00% | 100.00% | 91.71368%
Ordinary Income | Series B Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 100.00% | 100.00% | 91.71368%
Ordinary Income | Series C Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 100.00% | 91.71368%
Ordinary Income | Series D Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 100.00% | 100.00% | 0.00%
Return of Capital | Common Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Common stock, dividend rate (as percent) |  | 60.36811% | 71.18242% | 79.50018%
Return of Capital | Senior Common Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Common stock, dividend rate (as percent) |  | 0.00% | 0.00% | 0.00%
Return of Capital | Series A Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 0.00% | 0.00%
Return of Capital | Series B Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 0.00% | 0.00%
Return of Capital | Series C Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 0.00% | 0.00%
Return of Capital | Series D Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 0.00% | 0.00%
Long-Term Capital Gains | Common Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Common stock, dividend rate (as percent) |  | 0.00% | 0.00% | 1.69868%
Long-Term Capital Gains | Senior Common Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Common stock, dividend rate (as percent) |  | 0.00% | 0.00% | 8.28632%
Long-Term Capital Gains | Series A Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 0.00% | 8.28632%
Long-Term Capital Gains | Series B Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 0.00% | 8.28632%
Long-Term Capital Gains | Series C Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 0.00% | 8.28632%
Long-Term Capital Gains | Series D Preferred Stock |  |  |  |
Dividends Payable [Line Items] |  |  |  |
Preferred stock, dividend rate (as percent) |  | 0.00% | 0.00% | 0.00%

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