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Shareholders' Equity / Partners' Capital - Share Repurchase Program (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Feb. 23, 2018
Sep. 30, 2015
Class A common shares/units          
Class of Stock [Line Items]          
Repurchase of Class A common stock, authorized amount         $ 300,000,000
Weighted-average price of shares (in dollars per share)   $ 15.44      
Total cost of Class A common shares   $ 96,000,000      
Remaining repurchase authorization amount $ 146,700,000        
Class A common shares/units | Subsequent Events          
Class of Stock [Line Items]          
Repurchase of Class A common stock, authorized amount       $ 300,000,000  
Class A common shares/units | Common Stock          
Class of Stock [Line Items]          
Repurchase and retire of Class A common shares (in shares) 0 6,215,656 3,633,602    
Preferred shares | Subsequent Events          
Class of Stock [Line Items]          
Repurchase of Class A common stock, authorized amount       $ 250,000,000