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Shareholders' Equity / Partners' Capital - At the Market Common Share Offering Program (Details) - USD ($)
 3 Months Ended 12 Months Ended
Sep. 30, 2018
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2017
Class of Stock [Line Items]              
Proceeds from issuance of Class A common shares       $ 0 $ 694,765,000 $ 102,830,000  
Class A common shares/units              
Class of Stock [Line Items]              
Common stock, shares issued (in shares) 13,800,000 14,842,982 36,546,170        
Proceeds from issuance of Class A common shares $ 312,000,000 $ 336,500,000          
Stock issuance costs $ 9,200,000 $ 400,000   0 $ 10,637,000 227,000  
At the Market - Common Share Offering Program | Class A common shares/units              
Class of Stock [Line Items]              
Shares reserved for future issuance           $ 400,000,000  
Common stock, shares issued (in shares)         2,000,000 4,900,000  
Proceeds from issuance of Class A common shares         $ 46,200,000 $ 104,000,000  
Sale of stock, price per share (in dollars per share)         $ 22.74 $ 21.13  
Proceeds from sale of stock, net of issuance costs         $ 45,600,000 $ 102,800,000  
Stock issuance costs         $ 600,000 $ 1,200,000  
Capital shares amount authorized for future issuance             $ 500,000,000
Remaining available for future shares       $ 500,000,000