Investments Remaining Contractual Maturity of Secruity Lending Agreements (Details) - Overnight and Continuous - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	$ 59.6	$ 88.5
Securities Loaned and Securities Sold under Agreement to Repurchase, Gross Including Not Subject to Master Netting Arrangement	59.6	88.5
Secured Borrowings, Gross, Difference, Amount	0.0	0.0
United States Government and Government Agencies and Authorities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0.0	0.3
States, Municipalities, and Political Subdivisions
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0.0	0.0
Public Utilities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	2.2	6.3
All Other Corporate Bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	$ 57.4	$ 81.9

Amount of secured borrowings in excess of (less than) the liability for repurchase agreements and securities lending transactions, before offset, included in offsetting disclosures.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481420/860-30-50-7

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- Definition

Amount, before effects of master netting arrangements, of securities loaned to entities in exchange for collateral and funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-SubTopic 20
-Topic 210
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483466/210-20-50-3

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481420/860-30-50-7

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- Definition

Amount, before effects of master netting arrangements, of financial assets loaned to entities in exchange for collateral. Includes liabilities not subject to a master netting arrangement and financial assets transferred other than securities that qualify as secured borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481420/860-30-50-7

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section 50
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-Subparagraph (a)(1)
-SubTopic 30
-Topic 860
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481420/860-30-50-7

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481420/860-30-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481420/860-30-50-7

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