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/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/exampleRef
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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://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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The carrying amount as of the balance sheet date due the entity from (a) agents and insureds, (b) uncollected premiums and (c) others, net of the allowance for doubtful accounts.

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Premiums Receivable - Premiums Receivable and Agents' Balances (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	$ 5,652		$ 5,058
ACL	(117)	$ (119)	(109)	$ (108)	$ (107)	$ (105)
Premiums receivable and agents' balances, net of ACL	5,535		4,949
Premiums receivable, excluding receivables for losses within a deductible and retrospectively-rated policy premiums ("loss sensitive business")
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	5,281		4,698
ACL	(92)	(94)	(85)	(85)	(84)	(83)
Receivables for Loss Sensitive Business
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	371		360
ACL	(25)	$ (25)	(24)	$ (23)	$ (23)	$ (22)
Receivables for Loss Sensitive Business \| Standard & Poor's, AA Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	101		106
Receivables for Loss Sensitive Business \| Standard & Poor's, A Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	43		38
Receivables for Loss Sensitive Business \| Standard & Poor's, BBB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	127		119
Receivables for Loss Sensitive Business \| Standard & Poor's, BB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	60		56
Receivables for Loss Sensitive Business \| Standard & Poor's, Below BB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	$ 40		$ 41