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Transactions With Related Parties (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	Dec. 31, 2021
Related Party Transaction [Line Items]
Premiums ceded	$ 1,356	$ 1,167
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	6,612	5,941	$ 6,564	$ 5,881
Reinsurance balances payable	1,819		1,530
Net premiums written	3,424	2,634
Net premiums earned	$ 2,883	2,121
Greysbridge Holdings Ltd
Related Party Transaction [Line Items]
Percentage ownership	40.00%
Operating affiliates | Somers Holdings Ltd
Related Party Transaction [Line Items]
Premiums ceded	$ 219	221
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	1,200		1,200
Reinsurance balances payable	435		414
Equity Method Investee | Premia Holdings Ltd
Related Party Transaction [Line Items]
Net premiums written	75	1
Net premiums earned	75	$ 1
Funds Held under Reinsurance Agreements, Asset	$ 199		$ 119

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