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Transactions With Related Parties (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Related Party Transaction [Line Items]
Net premiums written	$ 4,085	$ 3,424
Net premiums earned	3,422	2,883
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	7,509	6,612	$ 7,064	$ 6,564
Reinsurance balances payable	2,497		2,000
Operating affiliates | Premia
Related Party Transaction [Line Items]
Net premiums written		75
Net premiums earned		75
Funds held asset	144		158
Operating affiliates | Somers
Related Party Transaction [Line Items]
Net premiums written	219	$ 155
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,400		1,300
Reinsurance balances payable	$ 584		$ 475

X

- Definition

Amount of the reinsurer's funds held on its behalf by the ceding insurer, including funds held in trust, under reinsurance treaties or contracts with the assuming insurer as security for the payment of the obligations thereunder.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

+ References

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written.

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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.

+ References

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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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