Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year		$ 67,811	$ 62,690	$ 62,960
Reinsurance recoverable on unpaid losses, beginning of year	[1]	(14,647)	(14,181)	(14,689)
Net unpaid losses and loss expenses, beginning of year		53,164	48,509	48,271
Net losses and loss expenses incurred in respect of losses incurring in Current Year		22,966	22,124	19,575
Prior Year Claims and Claims Adjustment Expense	[2]	(986)	(414)	(845)
Total		21,980	21,710	18,730
Net losses and loss expenses paid in Current Year		7,836	7,782	7,894
Net losses and loss expenses paid in Prior Year		10,048	9,652	10,579
Total		17,884	17,434	18,473
Foreign currency revaluation and other		(501)	379	(19)
Net unpaid losses and loss expenses, end of year		56,759	53,164	48,509
Reinsurance recoverable on unpaid losses, end of year	[1]	16,184	14,647	14,181
Gross unpaid losses and loss expenses, end of year		72,943	67,811	62,690
Prior Period Development, net adjustments		$ 60	$ 19	$ 53

[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 and earned premiums totaling $60 million, $19 million, and $53 million for 2021, 2020, and 2019, respectively.