Unpaid losses and loss expenses (RF) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 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		4,358	4,078
PPD, Gross	[2]	(256)	(289)
Total, Incurred		4,102	3,789
Net loss and loss expenses paid, Current Year		809	798
Net losses and loss expenses Paid, Prior Years		3,433	3,109
Total, Paid		4,242	3,907
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other		292	54
Net unpaid Loss, end of period		49,317	47,768
Reinsurance Recoverables on unpaid losses, end of period	[3]	13,822	12,811
Gross unpaid losses and loss expenses, end of period		63,139	60,579
prior period development, net adjustments		47	58
Reinsurance Recoverables, Allowance		$ 320	$ 334	$ 321	$ 300

[1] (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 $47 million and $58 million for the three months ended March 31, 2018 and 2017, respectively.

[3] 3) Net of provision for uncollectible reinsurance of $320 million and $334 million at March 31, 2018 and 2017, respectively.