Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Kudu Investment Management, LLC
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 219.3	$ 0.0
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	150.0	0.0
DavidShield & PassportCard
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	90.0	75.0
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	55.1	0.0
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	39.5	60.0
Kudu
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	0.0	30.7
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	553.9	165.7
Hedge and private equity funds included in other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	145.5	146.1
Insurance-linked securities funds
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	41.4	0.0
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	32.9	13.8
White Mountains
Debt and Equity Securities, FV-NI [Line Items]
Other Investments	$ 773.7	$ 325.6