Unpaid losses and loss expenses (RF) (Detail) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2019	Jun. 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		9,224	9,026
Prior Year Claims and Claims Adjustment Expense	[2]	(411)	(437)
Total, Incurred		8,813	8,589
Net loss and loss expenses paid, Current Year		2,288	2,346
Net loss and loss expenses paid, Prior Years		5,823	6,269
Total, Paid		8,111	8,615
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		(1)	(96)
Net unpaid losses and loss expenses – end of period		48,972	49,043
Reinsurance recoverable on unpaid losses (1)	[1]	14,233	13,735
Gross unpaid losses and loss expenses – end of period		63,205	62,778
prior period development, net adjustments		19	$ 37
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease), Gross		245
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)		$ 701

[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 $19 million and $37 million for the six months ended June 30, 2019 and 2018, respectively.