Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Investment securities
Other long-term investments	$ 1,377.8	$ 786.8
Kudu
Investment securities
Other long-term investments	669.5	400.6
PassportCard/DavidShield
Investment securities
Other long-term investments	120.0	95.0
Elementum Holdings L.P.
Investment securities
Other long-term investments	45.0	55.1
Other unconsolidated entities
Investment securities
Other long-term investments	34.4	42.4
Total unconsolidated entities
Investment securities
Other long-term investments	868.9	593.1
Bank loan fund
Investment securities
Other long-term investments	163.0	0.0
Private equity funds and hedge funds
Investment securities
Other long-term investments	153.8	121.2
Lloyd’s trust deposits
Investment securities
Other long-term investments	113.8	0.0
ILS funds
Investment securities
Other long-term investments	51.9	51.4
Private debt investments
Investment securities
Other long-term investments	14.1	21.1
Other
Investment securities
Other long-term investments	$ 12.3	$ 0.0