Related party transactions (Schedule of related party transactions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Related Party Transaction [Line Items]
Other income (expense)	$ 1,023	$ 836	$ (89)
Private equities	14,769	14,078
Ceded Premiums Written	10,535	10,165	10,258
Losses and loss expenses	26,022	24,100	22,572
Insurance and reinsurance balances payable	8,121	8,302
Total revenues	$ 55,753	49,735	43,097
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	$ 476	441	507
Commissions received	117	119	138
Reinsurance Recoverable on Losses and Loss Expenses	1,372	1,241
Insurance and reinsurance balances payable	112	40
BlackRock, Inc. | Related Party
Related Party Transaction [Line Items]
Total revenues	12	8	7
Aquiline Capital Partners LLC
Related Party Transaction [Line Items]
Investment Company, Financial Commitment to Investee, Future Amount	152
Other income (expense)	(60)	(36)	(8)
Private equities	400	368
Starr Technical Risk Agency and Affiliates [Member]
Related Party Transaction [Line Items]
Premiums Written, Gross	10	216	618
Ceded Premiums Written	24	115	353
Paid commissions	3	38	122
Commissions received	3	26	79
Losses and loss expenses	24	180	$ 225
Reinsurance Recoverable on Losses and Loss Expenses	328	503
Insurance and reinsurance balances payable	$ 19	$ 44