Investment Securities - Investments Held on Deposit or as Collateral (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Investments held in trusts	$ 517.5	$ 500.5
Assets held by insurance regulators	4.6	4.6
Other long-term investments	1,643.0	1,488.0
Outrigger Re \| Cash and Short Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Investment owned, balance	206.0	203.7
HG Global Senior Notes \| Secured Debt
Debt and Equity Securities, FV-NI [Line Items]
Interest reserve account	31.6	31.2
Lloyd’s trust deposits
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	148.7	137.4
Additional Investments, Insurance Regulators and Reinsurance Counterparties \| Asset Pledged as Collateral
Debt and Equity Securities, FV-NI [Line Items]
Investments pledged as collateral	263.3	257.0
Short-term investments \| Asset Pledged as Collateral
Debt and Equity Securities, FV-NI [Line Items]
Investments pledged as collateral	144.1	90.3
Lloyds Bank
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	$ 326.3	$ 319.2

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1A
-URI https://asc.fasb.org/extlink&oid=123600598&loc=SL75763979-209797

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+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(b))
-URI https://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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Investments Held In Trusts

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