v3.25.1
Investment Securities - Other Long-Term Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments $ 2,287.9 $ 2,150.2
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 64.7 63.6
Private equity funds and hedge funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 379.2 360.6
Bank loan fund    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 264.9 264.7
Lloyd’s trust deposits    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 155.9 149.9
ILS funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 71.3 74.0
Private debt instruments    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 15.3 14.9
Other    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 31.2 29.1
Kudu’s Participation Contracts | Asset Management (Kudu)    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 1,120.4 1,008.4
Investments, Excluding MediaAlpha | Total unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 1,370.1 1,257.0
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 586.8 547.8
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 134.8 104.1
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 264.9 264.7
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 155.9 149.9
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 31.2 29.1
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 1,126.2 1,014.0
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 5.8 5.6
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 1,120.4 1,008.4
Operating Segments | Asset Management (Kudu) | Investments, Excluding MediaAlpha | Total unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 1,120.4 1,008.4
Other Operations    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 574.9 588.4
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 64.7 63.6
Other Operations | Private equity funds and hedge funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 244.4 256.5
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 71.3 74.0
Other Operations | Private debt instruments    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 9.5 9.3
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 $ 249.7 $ 248.6