Unpaid losses and loss expenses (RF) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)		Sep. 30, 2017 USD ($)
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses – beginning of period	$ 63,179		$ 60,540
Reinsurance recoverable on unpaid losses - beginning of period (1)	(14,014)	[1]	(12,708)	[1]
Net unpaid losses and loss expenses – beginning of period	49,165		47,832
Current Year Claims and Claims Adjustment Expense	14,186		14,963
PPD, Gross	(729)	[2]	(781)	[2]
Total, Incurred	13,457		14,182
Net loss and loss expenses paid, Current Year	4,522		3,937
Net loss and loss expenses paid, Prior Years	8,596		8,389
Total, Paid	13,118		12,326
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other	(440)		596
Net unpaid losses and loss expenses – end of period	49,064		50,283
Reinsurance recoverable on unpaid losses (1)	13,965	[1]	13,870	[1]
Gross unpaid losses and loss expenses – end of period	63,029		64,153
prior period development, net adjustments	$ 86		$ 110

[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 $86 million and $110 million for the nine months ended September 30, 2018 and 2017, respectively.