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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2025	Dec. 31, 2024	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses		$ 9,044	$ 8,260	$ 8,969	$ 7,473	$ 7,509	$ 7,064
Reinsurance recoverable | Reinsurer concentration risk | AM Best A minus Or Better Rating
Ceded Credit Risk [Line Items]
Concentration risk percentage		62.50%	63.80%
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | All Other Carriers
Ceded Credit Risk [Line Items]
Concentration risk percentage	[1]	37.50%	36.20%
Reinsurance recoverable | Reinsurer concentration risk | Not Rated | Ceded credit risk, secured
Ceded Credit Risk [Line Items]
Concentration risk percentage		95.00%	95.00%
Stockholders' equity | Reinsurer concentration risk | Largest Balance Due From Any One Carrier
Ceded Credit Risk [Line Items]
Concentration risk percentage		7.90%	7.80%
[1] At June 30, 2025 and December 31, 2024 over 95% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other

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Amount, after valuation allowance, recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, policy benefits and policy reserves. Excludes premiums paid under reinsurance contracts.

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