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Summary of Significant Accounting Policies - Earnings per common share (Details) - shares
shares in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Earnings per common share      
Weighted average number of shares outstanding, basic 70.9 72.0 72.3
Weighted average number of shares outstanding, diluted 71.8 73.5 74.1
Antidilutive securities excluded from computation of earnings per share amount 0.5 0.3 0.0