Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024		Dec. 31, 2023		Dec. 31, 2022
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year	$ 80,122		$ 75,747		$ 72,330
Reinsurance recoverable on unpaid losses, beginning of year	(17,884)	[1]	(17,086)	[2]	(16,132)	[2]
Net unpaid losses and loss expenses, beginning of year	62,238		58,661		56,198
Net losses and loss expenses incurred in respect of losses occurring in Current Year	26,997		24,956		23,680
Prior Year Claims and Claims Adjustment Expense, net of PPD related to A&H	(975)		(856)	[3]	(1,108)	[3]
Total	26,022		24,100		22,572
Net losses and loss expenses paid in Current Year	8,681		8,248		7,331
Net losses and loss expenses paid in Prior Year	12,822		12,763		12,206
Total	21,503		21,011		19,537
Foreign currency revaluation and other	(487)		83		(572)
Net unpaid losses and loss expenses, end of year	66,270		62,238		58,661
Reinsurance recoverable on unpaid losses, end of year	17,734	[1]	17,884	[1]	17,086	[2]
Gross unpaid losses and loss expenses, end of year	84,004		80,122		75,747
Prior Period Development, net adjustments	119		83		232
Huatai Group [Member]
Unpaid Losses and Loss Expenses [Roll Forward]
Consolidation of Huatai Group	0		405		0
Liability for Claims and Claims Adjustment Expense [Line Items]
Consolidation of Huatai Group	$ 0		$ 405		$ 0

[1] Net of valuation allowance for uncollectible reinsurance.

[2] Net of valuation allowance for uncollectible reinsurance.

[3] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, earned premiums, and A&H long-duration lines totaling $119 million, $83 million, and $232 million for 2024, 2023, and 2022, respectively.