Premiums Receivable - Rollforward of Premiums Receivable and Agents' Balances (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Allowance for Credit Loss [Line Items]
Premium Receivable, Allowance for Credit Loss	$ (109)	$ (109)	$ (105)
Premium Receivable, Credit Loss Expense (Reversal)	65	50	51
Premium Receivable, Allowance for Credit Loss, Writeoff	63	57	57
Premium Receivable, Allowance for Credit Loss, Recovery	(6)	(7)	(10)
Premium Receivable, Allowance for Credit Loss	(117)	(109)	(109)
Premiums Receivable, Due in One Year or Less, Excluding Receivables for Loss within a Deductible and Retrospectively-Rated Policy Premiums [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Premium Receivable, Allowance for Credit Loss	(89)	(85)	(83)
Premium Receivable, Credit Loss Expense (Reversal)	64	52	48
Premium Receivable, Allowance for Credit Loss, Writeoff	62	55	56
Premium Receivable, Allowance for Credit Loss, Recovery	(6)	(7)	(10)
Premium Receivable, Allowance for Credit Loss	(97)	(89)	(85)
Premiums Receivable, Loss Within a Deductible and Retrospectively-Rated Policy Premiums [Member]
Financing Receivable, Allowance for Credit Loss [Line Items]
Premium Receivable, Allowance for Credit Loss	(20)	(24)	(22)
Premium Receivable, Credit Loss Expense (Reversal)	1	(2)	3
Premium Receivable, Allowance for Credit Loss, Writeoff	1	2	1
Premium Receivable, Allowance for Credit Loss, Recovery	0	0	0
Premium Receivable, Allowance for Credit Loss	$ (20)	$ (20)	$ (24)

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-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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- Definition

Amount of expense (reversal of expense) for expected credit loss on premium receivable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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- Definition

The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-4

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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- Definition

Amount of increase in allowance for credit loss on premium receivable from recovery.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

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- Definition

Amount of writeoff of premium receivable, charged against allowance for credit loss.

+ References

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