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Investment Securities - Other Long-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Investment securities
Other long-term investments	$ 2,150.2	$ 1,998.2
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	547.8	440.9
Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	1,014.0	896.3
Other Operations
Investment securities
Other long-term investments	588.4	661.0
PassportCard/DavidShield
Investment securities
Other long-term investments	150.0	150.0
PassportCard/DavidShield | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0.0	0.0
PassportCard/DavidShield | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0.0	0.0
PassportCard/DavidShield | Other Operations
Investment securities
Other long-term investments	150.0	150.0
Elementum
Investment securities
Other long-term investments	35.0	35.0
Elementum | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0.0	0.0
Elementum | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0.0	0.0
Elementum | Other Operations
Investment securities
Other long-term investments	35.0	35.0
Other unconsolidated entities
Investment securities
Other long-term investments	63.6	48.1
Other unconsolidated entities | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0.0	0.0
Other unconsolidated entities | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0.0	0.0
Other unconsolidated entities | Other Operations
Investment securities
Other long-term investments	63.6	48.1
Private equity funds and hedge funds
Investment securities
Other long-term investments	360.6	312.9
Private equity funds and hedge funds | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	104.1	55.5
Private equity funds and hedge funds | Operating Segments | Asset Management (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	256.5	257.4
Bank loan fund
Investment securities
Other long-term investments	264.7	194.4
Bank loan fund | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	264.7	194.4
Bank loan fund | Operating Segments | Asset Management (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
Lloyd’s trust deposits
Investment securities
Other long-term investments	149.9	158.0
Lloyd’s trust deposits | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	149.9	158.0
Lloyd’s trust deposits | Operating Segments | Asset Management (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
ILS funds
Investment securities
Other long-term investments	74.0	160.5
ILS funds | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0.0	0.0
ILS funds | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0.0	0.0
ILS funds | Other Operations
Investment securities
Other long-term investments	74.0	160.5
Private debt instruments
Investment securities
Other long-term investments	14.9	15.8
Private debt instruments | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0.0	0.0
Private debt instruments | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	5.6	5.8
Private debt instruments | Other Operations
Investment securities
Other long-term investments	9.3	10.0
Other
Investment securities
Other long-term investments	29.1	33.0
Other | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	29.1	33.0
Other | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	0.0	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	1,008.4	890.5
Kudu’s Participation Contracts | Kudu | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0.0	0.0
Kudu’s Participation Contracts | Kudu | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	1,008.4	890.5
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	1,257.0	1,123.6
Investments, Excluding MediaAlpha | Total unconsolidated entities | Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Investment securities
Other long-term investments	0.0	0.0
Investments, Excluding MediaAlpha | Total unconsolidated entities | Operating Segments | Asset Management (Kudu)
Investment securities
Other long-term investments	1,008.4	890.5
Investments, Excluding MediaAlpha | Total unconsolidated entities | Other Operations
Investment securities
Other long-term investments	$ 248.6	$ 233.1

X

- Definition

Amount of long-term investments classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1)(f))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(12))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1)(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6B
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-Publisher FASB
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