Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Investment securities
Other long-term investments	$ 1,488.0	$ 1,377.8
Kudu
Investment securities
Other long-term investments	695.9	669.5
PassportCard/DavidShield
Investment securities
Other long-term investments	135.0	120.0
Elementum Holdings L.P.
Investment securities
Other long-term investments	30.0	45.0
Other unconsolidated entities
Investment securities
Other long-term investments	37.2	34.4
Total unconsolidated entities
Investment securities
Other long-term investments	898.1	868.9
Private equity funds and hedge funds
Investment securities
Other long-term investments	197.8	153.8
Bank loan fund
Investment securities
Other long-term investments	174.8	163.0
Lloyd’s trust deposits
Investment securities
Other long-term investments	137.4	113.8
ILS funds
Investment securities
Other long-term investments	49.3	51.9
Private debt instruments
Investment securities
Other long-term investments	9.6	14.1
Other
Investment securities
Other long-term investments	$ 21.0	$ 12.3