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Shareholders' Equity - Perpetual Preferred Shares (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 3 Months Ended 6 Months Ended
Jul. 31, 2017
Jun. 30, 2017
Jun. 30, 2017
Jun. 30, 2016
Class of Stock [Line Items]        
Stock issuance costs     $ 350 $ 0
Underwritten Public Offering | Series F Perpetual Preferred Shares        
Class of Stock [Line Items]        
Shares issued during period (in shares)   6,200,000    
Preferred stock, dividend rate, percentage   5.875%    
Proceeds from issuance of preferred stock   $ 155,000    
Stock issuance costs   $ 5,200    
Liquidation preference per share (in dollars per share)   $ 25.00 $ 25.00  
Underwritten Public Offering | Series G, Preferred Perpetual Shares | Subsequent Event        
Class of Stock [Line Items]        
Shares issued during period (in shares) 4,600,000      
Preferred stock, dividend rate, percentage 5.875%      
Proceeds from issuance of preferred stock $ 115,000      
Stock issuance costs $ 4,100      
Liquidation preference per share (in dollars per share) $ 25.00