Unpaid losses and loss expenses (RF) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses – beginning of period		$ 67,811	$ 62,690
Reinsurance recoverable on unpaid losses – beginning of period (1)		(14,647)	(14,181)
Net unpaid losses and loss expenses – beginning of period		53,164	48,509
Current Year Claims and Claims Adjustment Expense		17,556	17,109
Prior Year Claims and Claims Adjustment Expense	[1]	(868)	(212)
Total, Incurred		16,688	16,897
Net loss and loss expenses paid, Current Year		4,771	4,766
Net loss and loss expenses paid, Prior Years		7,922	7,597
Total, Paid		12,693	12,363
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		(78)	95
Net unpaid losses and loss expenses – end of period		57,081	53,138
Reinsurance recoverable on unpaid losses (1)	[2]	15,550	14,767
Gross unpaid losses and loss expenses – end of period		72,631	67,905
prior period development, net adjustments		87	$ 23
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross		4,820
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)		$ 3,917

[1] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $87 million and $23 million for the nine months ended September 30, 2021 and 2020, respectively.

[2] Net of valuation allowance for uncollectible reinsurance.