Unpaid losses and loss expenses (RF) (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2020	Jun. 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)	[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		11,111	9,224
Prior Year Claims and Claims Adjustment Expense	[2]	(49)	(411)
Total, Incurred		11,062	8,813
Net loss and loss expenses paid, Current Year		2,263	2,288
Net loss and loss expenses paid, Prior Years		5,842	5,823
Total, Paid		8,105	8,111
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		(128)	(1)
Net unpaid losses and loss expenses – end of period		51,338	48,972
Reinsurance recoverable on unpaid losses (1)	[1]	14,361	14,233
Gross unpaid losses and loss expenses – end of period		65,699	63,205
prior period development, net adjustments		6	$ 19
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross		3,009
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)		$ 2,829

[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 $6 million and $19 million for the six months ended June 30, 2020 and 2019, respectively.