v3.26.3
Investment Securities - Other Long-Term Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments $ 3,417.5 $ 2,958.5
Bamboo SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 290.0 250.0
BroadStreet SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 176.8 160.0
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 134.0  
Other unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 123.0 108.7
Private equity funds and hedge funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 487.7 395.2
Bank loan fund    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 318.7 308.5
Lloyd’s trust deposits    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 185.0 180.4
Equity infrastructure fund    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 100.0  
ILS funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 54.9 50.1
Private debt instruments    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 19.9 16.9
Other    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 38.4 33.7
Kudu’s Participation Contracts | Asset Management (Kudu)    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 1,319.1 1,285.0
Investments, Excluding MediaAlpha | Total unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 2,212.9 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 868.0 689.7
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 225.9 167.1
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 318.7 308.5
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 185.0 180.4
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger) | Equity infrastructure fund    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 100.0  
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 38.4 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 1,319.1 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) | Equity infrastructure fund    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 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 0.0 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 1,319.1 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 1,319.1 1,285.0
Other Operations    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 1,230.4 977.4
Other Operations | Bamboo SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 290.0 250.0
Other Operations | BroadStreet SPV    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 176.8 160.0
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 134.0  
Other Operations | Other unconsolidated entities    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 123.0 108.7
Other Operations | Private equity funds and hedge funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 261.8 228.1
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 | Equity infrastructure fund    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 0.0  
Other Operations | ILS funds    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 54.9 50.1
Other Operations | Private debt instruments    
Debt and Equity Securities, FV-NI [Line Items]    
Other long-term investments 19.9 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 $ 893.8 $ 688.7