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Investment Securities - Other long-term investments (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	$ 1,815.0	$ 1,488.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	410.7	373.6
Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	737.1	695.9
Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	667.2	418.5
Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	737.1	695.9
Asset Management (Kudu) | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Asset Management (Kudu) | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	737.1	695.9
Asset Management (Kudu) | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
PassportCard/DavidShield
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	150.0	135.0
PassportCard/DavidShield | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
PassportCard/DavidShield | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
PassportCard/DavidShield | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	150.0	135.0
Elementum Holdings, L.P.
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	30.0	30.0
Elementum Holdings, L.P. | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Elementum Holdings, L.P. | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Elementum Holdings, L.P. | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	30.0	30.0
Other unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	87.5	37.2
Other unconsolidated entities | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Other unconsolidated entities | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Other unconsolidated entities | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	87.5	37.2
Total unconsolidated entities
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	1,004.6	898.1
Total unconsolidated entities | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Total unconsolidated entities | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	737.1	695.9
Total unconsolidated entities | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	267.5	202.2
Private equity funds and hedge funds
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	289.3	197.8
Private equity funds and hedge funds | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	52.8	40.4
Private equity funds and hedge funds | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Private equity funds and hedge funds | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	236.5	157.4
Bank loan fund
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	184.3	174.8
Bank loan fund | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	184.3	174.8
Bank loan fund | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Bank loan fund | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
ILS funds
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	153.3	49.3
ILS funds | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
ILS funds | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
ILS funds | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	153.3	49.3
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	152.3	137.4
Lloyd’s trust deposits | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	152.3	137.4
Lloyd’s trust deposits | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Lloyd’s trust deposits | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Private debt instruments
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	9.9	9.6
Private debt instruments | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Private debt instruments | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Private debt instruments | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	9.9	9.6
Other
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	21.3	21.0
Other | P&C Insurance and Reinsurance (Ark/WM Outrigger)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	21.3	21.0
Other | Asset Management (Kudu)
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	0.0	0.0
Other | Other Operations
Debt and Equity Securities, FV-NI [Line Items]
Total other long-term investments	$ 0.0	$ 0.0

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//1943274/2147479440/944-210-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

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