Equity-Method Eligible Investments (Carrying Values of Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Other Investments	$ 622.8	$ 165.7
White Mountains
Schedule of Equity Method Investments [Line Items]
Other Investments	856.3	325.6
White Mountains | Limited liability companies and private equity securities
Schedule of Equity Method Investments [Line Items]
Other Investments	761.7	269.7
White Mountains | Other unconsolidated investments
Schedule of Equity Method Investments [Line Items]
Other Investments	$ 94.6	$ 55.9