Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity Preferred Stock Issuances (Details)

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Stockholders' Equity Preferred Stock Issuances (Details) - USD ($)
$ / shares in Units, $ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Class of Stock [Line Items]      
Preferred stock par value per share (in usd per share) $ 0.01   $ 0.01
Proceeds from issuance of preferred stock, net of offering costs $ 13 $ 138,872  
Series A Preferred Stock [Member]      
Class of Stock [Line Items]      
Preferred stock dividend rate 8.125% 8.125%  
Preferred stock par value per share (in usd per share) $ 0.01    
Preferred stock public offering price per share (in usd per share) 25.00    
Preferred stock liquidation preference per share (in usd per share) $ 25.00    
Preferred stock dividend variable rate spread 5.66%    
Preferred stock redemption price per share (in usd per share) $ 25.00    
Proceeds from issuance of preferred stock, net of offering costs     $ 138,872
Series A Preferred Stock [Member] | Public Offering [Member]      
Class of Stock [Line Items]      
Number of shares of stock issued during period (in shares)     5,000,000
Series A Preferred Stock [Member] | Over-Allotment Option [Member]      
Class of Stock [Line Items]      
Number of shares of stock issued during period (in shares)     750,000
Series B Preferred Stock [Member]      
Class of Stock [Line Items]      
Number of shares of stock issued during period (in shares)     11,500,000
Preferred stock dividend rate 7.625% 7.625%  
Preferred stock par value per share (in usd per share) $ 0.01    
Preferred stock public offering price per share (in usd per share) 25.00    
Preferred stock liquidation preference per share (in usd per share) $ 25.00    
Preferred stock dividend variable rate spread 5.352%    
Preferred stock redemption price per share (in usd per share) $ 25.00    
Proceeds from issuance of preferred stock, net of offering costs     $ 278,094
Series B Preferred Stock [Member] | Over-Allotment Option [Member]      
Class of Stock [Line Items]      
Number of shares of stock issued during period (in shares)     1,500,000
Series C Preferred Stock [Member]      
Class of Stock [Line Items]      
Preferred stock dividend rate 7.25% 7.25%  
Preferred stock par value per share (in usd per share) $ 0.01    
Preferred stock public offering price per share (in usd per share) 25.00    
Preferred stock liquidation preference per share (in usd per share) $ 25.00    
Preferred stock dividend variable rate spread 5.011%    
Preferred stock redemption price per share (in usd per share) $ 25.00    
Proceeds from issuance of preferred stock, net of offering costs     $ 285,584
Series C Preferred Stock [Member] | Public Offering [Member]      
Class of Stock [Line Items]      
Number of shares of stock issued during period (in shares)     11,000,000
Series C Preferred Stock [Member] | Over-Allotment Option [Member]      
Class of Stock [Line Items]      
Number of shares of stock issued during period (in shares)     800,000