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Shareholders' Equity - At the Market Common Share Offering Program (Details) - USD ($)
$ / shares in Units, shares in Millions
 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Nov. 30, 2016
Class of Stock [Line Items]      
Proceeds from issuance of shares $ 355,589,000 $ 2,000  
Stock issuance costs 350,000 $ 0  
At the Market - Common Share Offering Program | Class A common shares      
Class of Stock [Line Items]      
Shares available for future issuance, value $ 276,600,000   $ 400,000,000
Shares issued during period (in shares) 0.9    
Proceeds from issuance of shares $ 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