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Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Concentration Risk [Line Items]
Reinsurance recoverable for paid and unpaid claims and claims adjustments		$ 8,260	$ 7,064	$ 6,564
Reinsurance recoverable | Reinsurer concentration risk | AM Best "A-" Or Better Rating
Concentration Risk [Line Items]
Concentration risk percentage		63.80%	66.80%
Reinsurance recoverable | Reinsurer concentration risk | Rating Below A- | All Other Carriers
Concentration Risk [Line Items]
Concentration risk percentage		0.00%	0.10%
Reinsurance recoverable | Reinsurer concentration risk | No AM Best rating | All Other Carriers
Concentration Risk [Line Items]
Concentration risk percentage	[1]	36.20%	33.10%
Reinsurance recoverable | Reinsurer concentration risk | No AM Best rating | Reinsurance trusts or letters of credit
Concentration Risk [Line Items]
Concentration risk percentage		95.00%	95.00%
Shareholders' equity | Reinsurer concentration risk | Largest balance due from any one carrier
Concentration Risk [Line Items]
Concentration risk percentage		7.80%	7.20%
[1] At December 31, 2024 and 2023 period, over 95% of such amount were collateralized through reinsurance trusts, funds withheld arrangements, letters of credit or other, respectively

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- Definition

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.

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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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