Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year		$ 75,747	$ 72,330		$ 67,192
Reinsurance recoverable on unpaid losses, beginning of year	[1]	(17,086)	(16,132)		(14,576)
Net unpaid losses and loss expenses, beginning of year		58,661	56,198		52,616
Net losses and loss expenses incurred in respect of losses occurring in Current Year		24,956	23,680		21,986
Prior Year Claims and Claims Adjustment Expense, net of PPD related to A&H		(856)	(1,108)	[2]	(956)	[2]
Prior Year Claims and Claims Adjustment Expense		(882)
Total		24,100	22,572		21,030
Net losses and loss expenses paid in Current Year		8,248	7,331		6,900
Net losses and loss expenses paid in Prior Year		12,763	12,206		10,048
Total		21,011	19,537		16,948
Foreign currency revaluation and other		83	(572)		(500)
Net unpaid losses and loss expenses, end of year		62,238	58,661		56,198
Reinsurance recoverable on unpaid losses, end of year	[1]	17,884	17,086		16,132
Gross unpaid losses and loss expenses, end of year		80,122	75,747		72,330
Prior Period Development, net adjustments		83	232		30
Huatai Group [Member]
Unpaid Losses and Loss Expenses [Roll Forward]
Consolidation of Huatai Group		405	0		0
Liability for Claims and Claims Adjustment Expense [Line Items]
Consolidation of Huatai Group		$ 405	$ 0		$ 0

[1] Net of valuation allowance for uncollectible reinsurance.

[2] 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 $83 million, $232 million, and $30 million for 2023, 2022, and 2021, respectively.