Unpaid losses and loss expenses (RF) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2025	Jun. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross unpaid losses and loss expenses – beginning of period		$ 84,004	$ 80,122
Reinsurance recoverable on unpaid losses and loss expenses - beginning of period	[1]	17,734	17,884
Net unpaid losses and loss expenses – beginning of period		66,270	62,238
Current Year Claims and Claims Adjustment Expense		13,959	12,604
Prior Year Claims and Claims Adjustment Expense	[2]	(491)	(446)
Total, Incurred		13,468	12,158
Net loss and loss expenses paid, Current Year		2,960	2,492
Net loss and loss expenses paid, Prior Years		8,982	6,918
Total, Paid		11,942	9,410
Foreign currency revaluation and other		889	(204)
Net unpaid losses and loss expenses – end of period		68,685	64,782
Reinsurance recoverable on unpaid losses and loss expenses - end of period	[1]	(17,691)	(17,409)
Gross unpaid losses and loss expenses – end of period		86,376	82,191
prior period development, net adjustments		$ (13)	$ 47

[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 $(13) million and $47 million for the six months ended June 30, 2025 and 2024, respectively.