SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2023 USD ($)
Summary of investments other than investments in related parties
Cost	$ 5,883.8
Carrying Value	6,388.7
Fair Value	6,388.7
Fixed maturity investments
Summary of investments other than investments in related parties
Cost	2,198.4
Carrying Value	2,109.3
Fair Value	2,109.3
U.S. Government and agency obligations
Summary of investments other than investments in related parties
Cost	209.0
Carrying Value	204.9
Fair Value	204.9
Debt securities issued by corporations
Summary of investments other than investments in related parties
Cost	1,085.9
Carrying Value	1,045.0
Fair Value	1,045.0
Municipal obligations
Summary of investments other than investments in related parties
Cost	275.1
Carrying Value	260.9
Fair Value	260.9
Mortgage and asset-backed securities
Summary of investments other than investments in related parties
Cost	417.2
Carrying Value	389.4
Fair Value	389.4
Collateralized loan obligations
Summary of investments other than investments in related parties
Cost	211.2
Carrying Value	209.1
Fair Value	209.1
Short-term investments
Summary of investments other than investments in related parties
Cost	1,487.0
Carrying Value	1,487.9
Fair Value	1,487.9
Equity Method Investments \| Investment in MediaAlpha
Summary of investments other than investments in related parties
Cost	59.2
Carrying Value	254.9
Fair Value	254.9
Common equity securities - Industrial, Miscellaneous, and Other
Summary of investments other than investments in related parties
Cost	370.2
Carrying Value	400.6
Fair Value	400.6
Exchange-traded funds
Summary of investments other than investments in related parties
Cost	113.3
Carrying Value	137.8
Fair Value	137.8
Total common equity securities
Summary of investments other than investments in related parties
Cost	483.5
Carrying Value	538.4
Fair Value	538.4
Other long-term investments
Summary of investments other than investments in related parties
Cost	1,655.7
Carrying Value	1,998.2
Fair Value	$ 1,998.2

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Subsection 15
-Publisher SEC

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480032/946-320-S99-7

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column D))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480032/946-320-S99-7

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column B))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480032/946-320-S99-7

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-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column C))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480032/946-320-S99-7

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