Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Kudu investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 262.7	$ 266.5
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	210.0	180.0
DavidShield & PassportCard
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	80.0	90.0
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	55.1	55.1
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	24.8	31.2
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	632.6	622.8
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	98.5	161.1
Insurance-linked securities funds
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	46.2	41.2
Private Debt Instrument
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	25.8	28.7
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	2.5	2.5
White Mountains
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 805.6	$ 856.3