Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity and Mezzanine Equity - Summary of Changes in Stockholders' Equity (Detail)

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Stockholders' Equity and Mezzanine Equity - Summary of Changes in Stockholders' Equity (Detail)
 1 Months Ended 12 Months Ended
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%