Unpaid losses and loss expenses (RF) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 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		5,249	4,605
Prior Year Claims and Claims Adjustment Expense	[1]	(196)	(120)
Total, Incurred		5,053	4,485
Net loss and loss expenses paid, Current Year		732	920
Net loss and loss expenses paid, Prior Years		3,161	3,335
Total, Paid		3,893	4,255
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		71	(565)
Net unpaid losses and loss expenses – end of period		54,395	48,174
Reinsurance recoverable on unpaid losses (1)	[2]	14,860	14,040
Gross unpaid losses and loss expenses – end of period		69,255	62,214
prior period development, net adjustments		4	$ 2
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross		1,444
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)		$ 1,231

[1] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $4 million and $2 million for the three months ended March 31, 2021 and 2020, respectively.

[2] Net of valuation allowance for uncollectible reinsurance.