Investment Securities - Other long-term investments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	$ 1,171.2	$ 856.3
Marketing Technology (MediaAlpha)
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	475.0	180.0
Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	325.5	266.5
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	95.0	90.0
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	55.1	55.1
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	31.8	33.7
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	982.4	625.3
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	118.2	161.1
Insurance-linked securities funds
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	52.7	41.2
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other Long-term Investments	17.9	28.7
Private debt investments	$ 5.1	$ 5.0