Investment Securities - Other long-term investments (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	$ 1,428.5	$ 1,377.8
Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	691.8	669.5
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	125.0	120.0
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	45.0	45.0
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	5.0	34.4
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	866.8	868.9
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	206.3	153.8
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	162.4	163.0
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	113.2	113.8
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	51.7	51.9
Private debt investments
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	11.5	14.1
Other
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	$ 16.6	$ 12.3