Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investment securities
Other long-term investments	$ 1,998.2	$ 1,488.0
Operating Segments | Ark
Investment securities
Other long-term investments	440.9	373.6
Operating Segments | Kudu
Investment securities
Other long-term investments	896.3	695.9
Other Operations
Investment securities
Other long-term investments	661.0	418.5
PassportCard/DavidShield
Investment securities
Other long-term investments	150.0	135.0
PassportCard/DavidShield | Operating Segments | Ark
Investment securities
Other long-term investments	0.0	0.0
PassportCard/DavidShield | Operating Segments | Kudu
Investment securities
Other long-term investments	0.0	0.0
PassportCard/DavidShield | Other Operations
Investment securities
Other long-term investments	150.0	135.0
Elementum Holdings, L.P.
Investment securities
Other long-term investments	35.0	30.0
Elementum Holdings, L.P. | Operating Segments | Ark
Investment securities
Other long-term investments	0.0	0.0
Elementum Holdings, L.P. | Operating Segments | Kudu
Investment securities
Other long-term investments	0.0	0.0
Elementum Holdings, L.P. | Other Operations
Investment securities
Other long-term investments	35.0	30.0
Other unconsolidated entities
Investment securities
Other long-term investments	48.1	37.2
Other unconsolidated entities | Operating Segments | Ark
Investment securities
Other long-term investments	0.0	0.0
Other unconsolidated entities | Operating Segments | Kudu
Investment securities
Other long-term investments	0.0	0.0
Other unconsolidated entities | Other Operations
Investment securities
Other long-term investments	48.1	37.2
Private equity funds and hedge funds
Investment securities
Other long-term investments	312.9	197.8
Private equity funds and hedge funds | Operating Segments | Ark
Investment securities
Other long-term investments	55.5	40.4
Private equity funds and hedge funds | Operating Segments | Kudu
Investment securities
Other long-term investments	0.0	0.0
Private equity funds and hedge funds | Other Operations
Investment securities
Other long-term investments	257.4	157.4
Bank loan fund
Investment securities
Other long-term investments	194.4	174.8
Bank loan fund | Operating Segments | Ark
Investment securities
Other long-term investments	194.4	174.8
Bank loan fund | Operating Segments | Kudu
Investment securities
Other long-term investments	0.0	0.0
Bank loan fund | Other Operations
Investment securities
Other long-term investments	0.0	0.0
ILS funds
Investment securities
Other long-term investments	160.5	49.3
ILS funds | Operating Segments | Ark
Investment securities
Other long-term investments	0.0	0.0
ILS funds | Operating Segments | Kudu
Investment securities
Other long-term investments	0.0	0.0
ILS funds | Other Operations
Investment securities
Other long-term investments	160.5	49.3
Lloyd’s trust deposits
Investment securities
Other long-term investments	158.0	137.4
Lloyd’s trust deposits | Operating Segments | Ark
Investment securities
Other long-term investments	158.0	137.4
Lloyd’s trust deposits | Operating Segments | Kudu
Investment securities
Other long-term investments	0.0	0.0
Lloyd’s trust deposits | Other Operations
Investment securities
Other long-term investments	0.0	0.0
Private debt instruments
Investment securities
Other long-term investments	10.0	9.6
Private debt instruments | Operating Segments | Ark
Investment securities
Other long-term investments	0.0	0.0
Private debt instruments | Operating Segments | Kudu
Investment securities
Other long-term investments	0.0	0.0
Private debt instruments | Other Operations
Investment securities
Other long-term investments	10.0	9.6
Other
Investment securities
Other long-term investments	38.8	21.0
Other | Operating Segments | Ark
Investment securities
Other long-term investments	33.0	21.0
Other | Operating Segments | Kudu
Investment securities
Other long-term investments	5.8	0.0
Other | Other Operations
Investment securities
Other long-term investments	0.0	0.0
Kudu’s Participation Contracts | Kudu
Investment securities
Other long-term investments	890.5	695.9
Kudu’s Participation Contracts | Kudu | Operating Segments | Ark
Investment securities
Other long-term investments	0.0	0.0
Kudu’s Participation Contracts | Kudu | Operating Segments | Kudu
Investment securities
Other long-term investments	890.5	695.9
Kudu’s Participation Contracts | Kudu | Other Operations
Investment securities
Other long-term investments	0.0	0.0
Investments, Excluding MediaAlpha | Total unconsolidated entities
Investment securities
Other long-term investments	1,123.6	898.1
Investments, Excluding MediaAlpha | Total unconsolidated entities | Operating Segments | Ark
Investment securities
Other long-term investments	0.0	0.0
Investments, Excluding MediaAlpha | Total unconsolidated entities | Operating Segments | Kudu
Investment securities
Other long-term investments	890.5	695.9
Investments, Excluding MediaAlpha | Total unconsolidated entities | Other Operations
Investment securities
Other long-term investments	$ 233.1	$ 202.2