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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, 2017
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Aug. 31, 2017
Nov. 30, 2016
Class of Stock [Line Items]                
Proceeds from issuance of Class A common shares       $ 694,765,000 $ 102,830,000 $ 0    
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   $ 10,637,000 $ 227,000 $ 0    
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