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Premiums Receivable - Premiums Receivable and Agents' Balances (Details) - USD ($) $ in Millions	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	$ 6,833		$ 6,115
ACL	(135)	$ (126)	(117)	$ (116)	$ (111)	$ (109)
Premiums receivable and agents' balances, net of ACL	6,698		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	6,347		5,624
ACL	(113)	(105)	(97)	(95)	(91)	(89)
Receivables for Loss Sensitive Business
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	486		491
ACL	(22)	$ (21)	(20)	$ (21)	$ (20)	$ (20)
Receivables for Loss Sensitive Business \| Standard & Poor's, AA Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	94		97
Receivables for Loss Sensitive Business \| Standard & Poor's, A Rating [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	56		57
Receivables for Loss Sensitive Business \| Standard & Poor's, BBB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	196		193
Receivables for Loss Sensitive Business \| Standard & Poor's, BB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	91		94
Receivables for Loss Sensitive Business \| Standard & Poor's, Below BB Rating
Financing Receivable, Credit Quality Indicator [Line Items]
Premiums Receivable, Gross	$ 49		$ 50