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Shareholders' Equity - Convertible non-voting common equivalent preferred shares (Details) - shares
 1 Months Ended 12 Months Ended
Dec. 31, 2016
Mar. 31, 2018
Jun. 30, 2017
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Class of Stock [Line Items]            
Shares issued [1]       2,646,164 2,835,994 2,757,506
Number of shares converted [2]       0 0 17,022,600
Series D Preferred Shares            
Class of Stock [Line Items]            
Number of share in conversion       10    
Number of underlying common shares 38,288,460          
Shares issued   17,022,600 21,265,860      
Number of shares converted   567,420 708,862      
Number of preferred shares outstanding       0    
United Guaranty Corporation | Series D Preferred Shares            
Class of Stock [Line Items]            
Number of shares issued 1,276,282          
[1] Includes shares issued from the exercise of stock options and stock appreciation rights, the vesting of restricted share units and shares issued from the employee share purchase plan.
[2] Such shares represent common shares that were issued upon conversion of the non-voting common equivalent preference shares issued in connection with the AIG acquisition.