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Shareholders' Equity / Partners' Capital - At the Market Common Share Offering Program (Details) - USD ($)
$ / shares in Units, shares in Millions
 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Aug. 31, 2017
Nov. 30, 2016
Class of Stock [Line Items]        
Proceeds from issuance of shares/units $ 0 $ 355,589,000    
Class A common shares/units        
Class of Stock [Line Items]        
Stock issuance costs 0 $ 350,000    
At the Market - Common Share Offering Program | Class A common shares/units        
Class of Stock [Line Items]        
Shares available for future issuance, value $ 500,000,000     $ 400,000,000
Shares issued during period (in shares)   0.9    
Proceeds from issuance of shares/units   $ 19,400,000    
Shares issued, price per share (in dollars per share)   $ 22.75    
Proceeds from issuance of common stock, net of issuance costs   $ 19,100,000    
Stock issuance costs   $ 300,000    
Shares authorized for future issuance, value     $ 500,000,000