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Premiums Receivable - Premiums Receivable and Agents' Balances (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	$ 6,693		$ 6,115
ACL	(138)	$ (135)	(117)	$ (117)	$ (116)	$ (109)
Premiums receivable and agents' balances, net of ACL	6,555		5,998
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	6,202		5,624
ACL	(115)	(113)	(97)	(96)	(95)	(89)
Receivables for Loss Sensitive Business
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	491		491
ACL	(23)	$ (22)	(20)	$ (21)	$ (21)	$ (20)
Receivables for Loss Sensitive Business \| Standard & Poor's, AA Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	93		97
Receivables for Loss Sensitive Business \| Standard & Poor's, A Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	58		57
Receivables for Loss Sensitive Business \| Standard & Poor's, BBB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	200		193
Receivables for Loss Sensitive Business \| Standard & Poor's, BB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	92		94
Receivables for Loss Sensitive Business \| Standard & Poor's, Below BB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	$ 48		$ 50