Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Kudu investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 315.6	$ 266.5
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	225.0	180.0
DavidShield & PassportCard
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	95.0	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	31.9	33.7
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	722.6	625.3
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	105.9	161.1
Insurance-linked securities funds
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	52.1	41.2
Private Debt Instrument
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	17.5	28.7
Noblr [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	4.6	5.0
White Mountains
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 898.1	$ 856.3