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Summary of Significant Accounting Policies - Earnings Per Share (Details) - shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]      
Number of shares of Common stock received for each exchangeable share (in shares) 1    
Weighted average shares outstanding attributable to participating securities (in shares) 2,600,000 2,300,000 2,600,000