Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 2,071.1	$ 1,998.2
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.0	150.0
Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35.0	35.0
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	56.6	48.1
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	349.0	312.9
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	258.9	194.4
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	172.7	158.0
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	71.2	160.5
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	15.8	15.8
Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	31.3	33.0
Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	930.6	890.5
Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,172.2	1,123.6
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	544.8	440.9
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	81.9	55.5
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	258.9	194.4
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	172.7	158.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	31.3	33.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	936.8	896.3
Operating Segments | Asset Management (Kudu) | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu) | Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu) | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu) | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu) | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu) | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu) | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu) | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	6.2	5.8
Operating Segments | Asset Management (Kudu) | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Operating Segments | Asset Management (Kudu) | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	930.6	890.5
Operating Segments | Asset Management (Kudu) | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	930.6	890.5
Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	589.5	661.0
Other Operations | PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.0	150.0
Other Operations | Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35.0	35.0
Other Operations | Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	56.6	48.1
Other Operations | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	267.1	257.4
Other Operations | Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Other Operations | Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Other Operations | ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	71.2	160.5
Other Operations | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	9.6	10.0
Other Operations | Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Other Operations | Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Other Operations | Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 241.6	$ 233.1