Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 107,472	$ 86,664
Reinsurance recoverable on unpaid losses at beginning of year	(5,025)	(2,960)
Net liabilities at beginning of year	102,447	83,704
Incurred losses and loss adjustment expenses, current accident year	29,827	27,427
Incurred losses and loss adjustment expenses, prior accident years	(1,948)	(887)
Total incurred losses and loss adjustment expenses	27,879	26,540
Paid losses and loss adjustment expenses, current accident year	(9,968)	(10,085)
Paid losses and loss adjustment expenses, prior accident years	(16,664)	(13,820)
Total paid losses and loss adjustment expenses	(26,632)	(23,905)
Foreign currency effect	267	(568)
Net liabilities at end of period	103,961	85,771
Reinsurance recoverable on unpaid losses at end of period	5,069	2,789
Gross liabilities at end of period	$ 109,030	$ 88,560