Related party transactions (Schedule of related party transactions) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Related Party Transaction [Line Items]
Other (income) expense	$ (836)	$ 89	$ (2,367)
Private equities	14,078	12,355
Ceded Premiums Written	10,165	10,258	8,912
Losses and loss expenses	24,100	22,572	21,030
Insurance and reinsurance balances payable	8,302	7,780
Total revenues	$ 49,735	$ 43,097	40,869
Bermuda \| ABR Reinsurance Capital Holdings Ltd. [Member]
Related Party Transaction [Line Items]
Ownership Percentage	19.00%	19.00%
Bermuda \| ABR Reinsurance Capital Holdings Ltd. [Member] \| ABR Reinsurance Capital Holdings Ltd. [Member]
Related Party Transaction [Line Items]
Ownership Percentage	18.70%
ABR Reinsurance Capital Holdings Ltd. [Member]
Related Party Transaction [Line Items]
Warrants & Rights Outstanding	0.50%
Ceded Premiums Written	$ 441	$ 507	442
Commissions received	119	138	133
Reinsurance Recoverable on Losses and Loss Expenses	1,241	1,050
Insurance and reinsurance balances payable	40	110
BlackRock, Inc. \| Related Party
Related Party Transaction [Line Items]
Total revenues	8	7	11
Aquiline Capital Partners LLC
Related Party Transaction [Line Items]
Investment Company, Financial Commitment to Investee, Future Amount	182
Other (income) expense	36	8	68
Private equities	368	271
Starr Technical Risk Agency and Affiliates [Member]
Related Party Transaction [Line Items]
Premiums Written, Gross	216	618	592
Ceded Premiums Written	115	353	321
Paid commissions	38	122	114
Commissions received	26	79	73
Losses and loss expenses	180	225	$ 157
Reinsurance Recoverable on Losses and Loss Expenses	503	541
Insurance and reinsurance balances payable	$ 44	$ 96

Reinsurance Recoverable on Losses and Loss Expenses

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Value of outstanding derivative securities that permit the holder the right to purchase a percentage of equity from the issuer.

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

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The expense incurred by an insurance company to persons or entities for generating or placing insurance or investment contracts with the company, usually determined as a percentage of policy or contract premiums. Excludes advances or draws to be applied against commissions earned.

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Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products.

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Amount of financial support committed by investment company to investee that is contractually required but not provided.

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Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.

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Amount of direct and assumed premiums written.

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The carrying amount as of the balance sheet date of the known and estimated amounts owed to insurers under reinsurance treaties or other arrangements.

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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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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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