Annual report pursuant to Section 13 and 15(d)

Mandatorily-Redeemable Preferred Stock - Characterization of Dividends Paid (Details)

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Mandatorily-Redeemable Preferred Stock - Characterization of Dividends Paid (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dividends Payable [Line Items]      
Ordinary Income 26.8429% 30.65818% 62.2954%
Return of Capital 67.39436% 69.34182% 37.34781%
Long-term Capital Gain 5.76274% 0.00% 0.35679%
Preferred Stock | Series A cumulative term preferred stock      
Dividends Payable [Line Items]      
Ordinary Income 82.32594% 100.00%  
Return of Capital 0.00% 0.00%  
Long-term Capital Gain 17.67406% 0.00%