Investment Securities - Other long-term investments (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	$ 1,643.0	$ 1,488.0
Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	683.2	695.9
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	140.0	135.0
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	30.0	30.0
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	87.5	37.2
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	940.7	898.1
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	218.5	197.8
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	179.5	174.8
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	148.7	137.4
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	123.6	49.3
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	10.1	9.6
Other
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	$ 21.9	$ 21.0