Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 68,458	$ 61,122	$ 53,379
Reinsurance recoverable on unpaid losses at beginning of year	(3,060)	(3,201)	(3,338)
Net liabilities at beginning of year	65,398	57,921	50,041
Net liabilities at end of year	70,164	65,398	57,921
Reinsurance recoverable on unpaid losses at end of year	2,855	3,060	3,201
Gross liabilities at end of year	73,019	68,458	61,122
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	43,335	39,876	37,702
Incurred losses and loss adjustment expenses, prior accident years' events	(752)	(1,406)	(544)
Total incurred losses and loss adjustment expenses	42,583	38,470	37,158
Paid losses and loss adjustment expenses, current accident year events	(19,482)	(18,391)	(17,425)
Paid losses and loss adjustment expenses, prior accident years' events	(17,642)	(15,452)	(12,507)
Total payments	(37,124)	(33,843)	(29,932)
Foreign currency translation adjustment	(23)	(331)	$ 654
Business acquisition (disposition)	$ (670)	$ 3,181