Unpaid losses and loss expenses (RF) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 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		4,605	4,326
Prior Year Claims and Claims Adjustment Expense	[2]	(120)	(228)
Total, Incurred		4,485	4,098
Net loss and loss expenses paid, Current Year		920	785
Net loss and loss expenses paid, Prior Years		3,335	3,234
Total, Paid		4,255	4,019
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		(565)	86
Net unpaid losses and loss expenses – end of period		48,174	48,436
Reinsurance recoverable on unpaid losses (1)	[1]	14,040	14,707
Gross unpaid losses and loss expenses – end of period		62,214	63,143
prior period development, net adjustments		2	$ 24
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross		(476)
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)		$ (335)

[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 $2 million and $24 million for the three months ended March 31, 2020 and 2019, respectively.