Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 79,854	$ 73,019
Reinsurance recoverable on unpaid losses at beginning of year	(2,912)	(2,855)
Net liabilities at beginning of year	76,942	70,164
Net liabilities at end of period	79,985	72,760
Reinsurance recoverable on unpaid losses at end of period	2,973	2,909
Gross liabilities at end of period	82,958	75,669
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	23,651	20,278
Incurred losses and loss adjustment expenses, prior accident years' events	(1,064)	189
Total	22,587	20,467
Paid losses and loss adjustment expenses, current accident year events	(8,406)	(6,638)
Paid losses and loss adjustment expenses, prior accident years' events	(11,169)	(11,155)
Total	(19,575)	(17,793)
Foreign currency translation adjustment	$ 31	$ (78)