Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 1,991.9	$ 1,998.2
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.0	150.0
Elementum Holdings, L.P.
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	47.1	48.1
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	317.8	312.9
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	249.6	194.4
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	150.5	158.0
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	109.7	160.5
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	15.6	15.8
Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	32.4	33.0
Kudu’s Participation Contracts | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	884.2	890.5
Investments, Excluding MediaAlpha | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,116.3	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	485.0	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 Holdings, L.P.
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	52.5	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	249.6	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	150.5	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	32.4	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	889.9	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 Holdings, L.P.
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	5.7	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	884.2	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	884.2	890.5
Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	617.0	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 Holdings, L.P.
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	47.1	48.1
Other Operations | Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	265.3	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	109.7	160.5
Other Operations | Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	9.9	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	$ 232.1	$ 233.1