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
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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.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
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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	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	$ 6,099	$ 5,716
ACL	(111)	(109)	$ (111)	$ (109)
Premiums receivable and agents' balances, net of ACL	5,988	5,607
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	5,676	5,303
ACL	(91)	(89)	(86)	(85)
Receivables for Loss Sensitive Business
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	423	413
ACL	(20)	(20)	$ (25)	$ (24)
Receivables for Loss Sensitive Business \| Standard & Poor's, AA Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	93	94
Receivables for Loss Sensitive Business \| Standard & Poor's, A Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	57	54
Receivables for Loss Sensitive Business \| Standard & Poor's, BBB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	140	136
Receivables for Loss Sensitive Business \| Standard & Poor's, BB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	88	84
Receivables for Loss Sensitive Business \| Standard & Poor's, Below BB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	$ 45	$ 45