Unpaid losses and loss expenses (RF) (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses – beginning of period		$ 62,960	$ 63,179
Reinsurance recoverable on unpaid losses - beginning of period (1)	[1]	(14,689)	(14,014)
Net unpaid losses and loss expenses – beginning of period		48,271	49,165
Current Year Claims and Claims Adjustment Expense		14,484	14,186
Prior Year Claims and Claims Adjustment Expense	[2]	(619)	(729)
Total, Incurred		13,865	13,457
Net loss and loss expenses paid, Current Year		4,920	4,522
Net loss and loss expenses paid, Prior Years		8,374	8,596
Total, Paid		13,294	13,118
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		(162)	(440)
Net unpaid losses and loss expenses – end of period		48,680	49,064
Reinsurance recoverable on unpaid losses (1)	[1]	14,332	13,965
Gross unpaid losses and loss expenses – end of period		63,012	63,029
prior period development, net adjustments		60	$ 86
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross		52
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)		$ 409

[1] Net of provision 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 and $86 million for the nine months ended September 30, 2019 and 2018, respectively.