Unpaid losses and loss expenses (RF) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2025	Mar. 31, 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)	[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		7,133	5,980
Prior Year Claims and Claims Adjustment Expense	[2]	(237)	(253)
Total, Incurred		6,896	5,727
Net loss and loss expenses paid, Current Year		1,305	767
Net loss and loss expenses paid, Prior Years		4,699	4,055
Total, Paid		6,004	4,822
Foreign currency revaluation and other		228	35
Net unpaid losses and loss expenses – end of period		67,390	63,178
Reinsurance recoverable on unpaid losses and loss expenses (1)	[1]	18,081	17,163
Gross unpaid losses and loss expenses – end of period		85,471	80,341
prior period development, net adjustments		$ (18)	$ 46

[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 $(18) million and $46 million for the three months ended March 31, 2025 and 2024, respectively.