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	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at the beginning of the year	$ 111,082	$ 107,472
Reinsurance recoverable on unpaid losses at the beginning of the year	(4,893)	(5,025)
Net liabilities at the beginning of the year	106,189	102,447
Incurred losses and loss adjustment expenses, current accident year	43,166	44,537
Incurred losses and loss adjustment expenses, prior accident years	(992)	(3,126)
Total incurred losses and loss adjustment expenses	42,174	41,411
Paid losses and loss adjustment expenses, current accident year	(16,011)	(16,962)
Paid losses and loss adjustment expenses, prior accident years	(21,370)	(22,171)
Total paid losses and loss adjustment expenses	(37,381)	(39,133)
Foreign currency effect	206	68
Net liabilities at end of period	111,188	104,793
Reinsurance recoverable on unpaid losses at end of period	4,648	5,031
Gross liabilities at end of period	$ 115,836	$ 109,824