v3.26.1
Investment Securities - Other Long-Term Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments $ 3,253.5 $ 2,958.5
Bamboo SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 260.0 250.0
BroadStreet SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 170.1 160.0
PassportCard/DavidShield    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 170.0 170.0
Bishop Street    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 125.0  
Other unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 113.5 108.7
Private equity funds and hedge funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 476.8 395.2
Bank loan fund    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 307.8 308.5
Lloyd’s trust deposits    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 176.4 180.4
ILS funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 51.0 50.1
Private debt instruments    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 16.8 16.9
Other    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 33.9 33.7
Kudu’s Participation Contracts | Asset Management (Kudu)    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 1,352.2 1,285.0
Investments, Excluding MediaAlpha | Total unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 2,190.8 1,973.7
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 737.7 689.7
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Bamboo SPV    
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) | BroadStreet SPV    
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) | 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) | Bishop Street    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 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 219.6 167.1
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 307.8 308.5
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 176.4 180.4
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 33.9 33.7
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 1,358.8 1,291.4
Operating Segments | Asset Management (Kudu) | Bamboo SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 0.0 0.0
Operating Segments | Asset Management (Kudu) | BroadStreet SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 0.0 0.0
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) | Bishop Street    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 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 6.6 6.4
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 1,352.2 1,285.0
Operating Segments | Asset Management (Kudu) | Investments, Excluding MediaAlpha | Total unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 1,352.2 1,285.0
Other Operations    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 1,157.0 977.4
Other Operations | Bamboo SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 260.0 250.0
Other Operations | BroadStreet SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 170.1 160.0
Other Operations | PassportCard/DavidShield    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 170.0 170.0
Other Operations | Bishop Street    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 125.0  
Other Operations | Other unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 113.5 108.7
Other Operations | Private equity funds and hedge funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 257.2 228.1
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 51.0 50.1
Other Operations | Private debt instruments    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 10.2 10.5
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 $ 838.6 $ 688.7