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Equity - Redemption of Preferred Stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Apr. 30, 2017
Dec. 31, 2017
Equity    
Excess of redemption value over carrying value of preferred shares subject to redemption   $ 13,373
Class F Preferred Stock    
Equity    
Preferred stock redeemed 16,350,000  
Preferred stock, dividend rate (as a percent) 6.625%  
Preferred stock, redemption price per share (in dollars per share) $ 25  
Excess of redemption value over carrying value of preferred shares subject to redemption   $ 13,400