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Shareholders' Equity / Partners' Capital - Perpetual Preferred Shares (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2017
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2017
Dec. 31, 2016
Class of Stock [Line Items]          
Liquidation preference per share (in dollars per share)       $ 25  
Preferred stock, change in control redemption period       120 days  
Preferred shares, shares issued (in shares)       38,350,000 37,010,000
Series D Perpetual Preferred Shares/Units          
Class of Stock [Line Items]          
Preferred Shares dividend rate     6.50% 6.50%  
Preferred shares, shares issued (in shares)     10,750,000    
Gross proceeds from issuance of Preferred shares before offering costs     $ 268.8    
Adjustments to additional paid in capital, stock issued, issuance costs     $ 8.5    
Series E Perpetual Preferred Stock          
Class of Stock [Line Items]          
Preferred Shares dividend rate       6.35%  
Preferred shares, shares issued (in shares)     9,200,000    
Gross proceeds from issuance of Preferred shares before offering costs     $ 230.0    
Adjustments to additional paid in capital, stock issued, issuance costs     $ 7.5    
Series F Perpetual Preferred Stock          
Class of Stock [Line Items]          
Preferred Shares dividend rate       5.875%  
Liquidation preference per share (in dollars per share)   $ 25.00      
Issuance of perpetual preferred shares, net of offering costs (in shares)   6,200,000      
Proceeds from issuance of redeemable preferred stock   $ 155.0      
Offering costs   $ 5.3      
Series G Perpetual Preferred Stock          
Class of Stock [Line Items]          
Preferred Shares dividend rate       5.875%  
Liquidation preference per share (in dollars per share) $ 25.00        
Issuance of perpetual preferred shares, net of offering costs (in shares) 4,600,000        
Proceeds from issuance of redeemable preferred stock $ 115.0        
Offering costs $ 4.1