Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 2,958.5	$ 2,150.2
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	689.7	547.8
Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,291.4	1,014.0
Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	977.4	588.4
Bamboo SPV
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	250.0
Bamboo SPV | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0
Bamboo SPV | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0
Bamboo SPV | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	250.0
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	170.0	150.0
PassportCard/DavidShield | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
PassportCard/DavidShield | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
PassportCard/DavidShield | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	170.0	150.0
BroadStreet SPV
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	160.0
BroadStreet SPV | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0
BroadStreet SPV | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0
BroadStreet SPV | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	160.0
Elementum
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	35.0	35.0
Elementum | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Elementum | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Elementum | Other Operations
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	73.7	63.6
Other unconsolidated entities | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Other unconsolidated entities | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Other unconsolidated entities | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	73.7	63.6
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	395.2	360.6
Private equity funds and hedge funds | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	167.1	104.1
Private equity funds and hedge funds | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Private equity funds and hedge funds | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	228.1	256.5
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	308.5	264.7
Bank loan fund | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	308.5	264.7
Bank loan fund | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Bank loan fund | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	180.4	149.9
Lloyd’s trust deposits | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	180.4	149.9
Lloyd’s trust deposits | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Lloyd’s trust deposits | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	50.1	74.0
ILS funds | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
ILS funds | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
ILS funds | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	50.1	74.0
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	16.9	14.9
Private debt instruments | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Private debt instruments | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	6.4	5.6
Private debt instruments | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	10.5	9.3
Other
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	33.7	29.1
Other | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	33.7	29.1
Other | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Other | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Kudu’s Participation Contracts | Kudu
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,285.0	1,008.4
Kudu’s Participation Contracts | Kudu | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Kudu’s Participation Contracts | Kudu | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,285.0	1,008.4
Kudu’s Participation Contracts | Kudu | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Total unconsolidated entities | Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,973.7	1,257.0
Total unconsolidated entities | Total unconsolidated entities | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Total unconsolidated entities | Total unconsolidated entities | Operating Segments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,285.0	1,008.4
Total unconsolidated entities | Total unconsolidated entities | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 688.7	$ 248.6