Unpaid losses and loss expenses (RF) (Detail) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
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)	(12,708)
Net unpaid loss, beginning of period		49,165	47,832
Current Year Claims and Claims Adjustment Expense		9,026	8,396
PPD, Gross	[2]	(437)	(461)
Total, Incurred		8,589	7,935
Net loss and loss expenses paid, Current Year		2,346	2,271
Net losses and loss expenses Paid, Prior Years		6,269	5,758
Total, Paid		8,615	8,029
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		(96)	171
Net unpaid Loss, end of period		49,043	47,909
Reinsurance Recoverables on unpaid losses, end of period	[3]	13,735	12,485
Gross unpaid losses and loss expenses, end of period		62,778	60,394
Reinsurance Recoverables, Allowance		319	335	$ 321	$ 300
prior period development, net adjustments		$ 37	$ 60

[1] Net of provision for uncollectible reinsurance of $321 million and $300 million at December 31, 2017 and 2016, respectively.

[2] Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, and earned premiums totaling $37 million and $60 million for the six months ended June 30, 2018 and 2017, respectively.

[3] Net of provision for uncollectible reinsurance of $319 million and $335 million at June 30, 2018 and 2017, respectively.