Unpaid losses and loss expenses (RF) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses – beginning of period		$ 62,690	$ 62,960
Reinsurance recoverable on unpaid losses - beginning of period (1)		(14,181)	(14,689)
Net unpaid losses and loss expenses – beginning of period		48,509	48,271
Current Year Claims and Claims Adjustment Expense		17,109	14,484
Prior Year Claims and Claims Adjustment Expense	[1]	(212)	(619)
Total, Incurred		16,897	13,865
Net loss and loss expenses paid, Current Year		4,766	4,920
Net loss and loss expenses paid, Prior Years		7,597	8,374
Total, Paid		12,363	13,294
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		95	(162)
Net unpaid losses and loss expenses – end of period		53,138	48,680
Reinsurance recoverable on unpaid losses (1)	[2]	14,767	14,332
Gross unpaid losses and loss expenses – end of period		67,905	63,012
prior period development, net adjustments		23	$ 60
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross		5,215
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)		$ 4,629

[1] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $23 million and $60 million for the nine months ended September 30, 2020 and 2019, respectively.

[2] Net of valuation allowance for uncollectible reinsurance.