Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 73,019	$ 68,458	$ 61,122
Reinsurance recoverable on unpaid losses at beginning of year	(2,855)	(3,060)	(3,201)
Net liabilities at beginning of year	70,164	65,398	57,921
Net liabilities at end of year	76,942	70,164	65,398
Reinsurance recoverable on unpaid losses at end of year	2,912	2,855	3,060
Gross liabilities at end of year	79,854	73,019	68,458
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year events	43,400	43,335	39,876
Incurred losses and loss adjustment expenses, prior accident years' events	(356)	(752)	(1,406)
Total	43,044	42,583	38,470
Paid losses and loss adjustment expenses, current accident year events	(17,884)	(19,482)	(18,391)
Paid losses and loss adjustment expenses, prior accident years' events	(18,862)	(17,642)	(15,452)
Total	(36,746)	(37,124)	(33,843)
Foreign currency translation adjustment	$ 480	(23)	(331)
Business acquisition (disposition)		$ (670)	$ 3,181