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Equity-Method Eligible Investments - Equity Method Investments Financial Data (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]      
Total assets $ 6,900.7   $ 4,831.4
Total liabilities 2,964.8   1,013.5
Total revenues 178.2 $ (59.2)  
Net loss attributable to White Mountains’s common shareholders (75.3) (128.8)  
MediaAlpha, Kudu and Tuckerman      
Schedule of Equity Method Investments [Line Items]      
Total assets 241.7   212.7
Total liabilities 331.1   $ 315.8
Total revenues 173.6 119.4  
Total expenses 173.4 110.6  
Net loss attributable to White Mountains’s common shareholders $ 0.2 $ 8.8