Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year		$ 62,690	$ 62,960	$ 63,179
Reinsurance recoverable on unpaid losses, beginning of year	[1]	(14,181)	(14,689)	(14,014)
Net unpaid losses and loss expenses, beginning of year		48,509	48,271	49,165
Net losses and loss expenses incurred in respect of losses incurring in Current Year		22,124	19,575	19,048
Prior Year Claims and Claims Adjustment Expense	[2]	(414)	(845)	(981)
Total		21,710	18,730	18,067
Net losses and loss expenses paid in Current Year		7,782	7,894	7,544
Net losses and loss expenses paid in Prior Year		9,652	10,579	10,796
Total		17,434	18,473	18,340
Foreign currency revaluation and other		379	(19)	(621)
Net unpaid losses and loss expenses, end of year		53,164	48,509	48,271
Reinsurance recoverable on unpaid losses, end of year	[1]	14,647	14,181	14,689
Gross unpaid losses and loss expenses, end of year		67,811	62,690	62,960
Prior Period Development, net adjustments		$ 19	$ 53	$ 85

[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 $19 million, $53 million, and $85 million for 2020, 2019, and 2018, respectively.