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	3 Months Ended
	Mar. 31, 2024	Mar. 31, 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	13,854	14,776
Incurred losses and loss adjustment expenses, prior accident years	(634)	(740)
Total incurred losses and loss adjustment expenses	13,220	14,036
Paid losses and loss adjustment expenses, current accident year	(3,663)	(3,841)
Paid losses and loss adjustment expenses, prior accident years	(8,979)	(9,747)
Total paid losses and loss adjustment expenses	(12,642)	(13,588)
Foreign currency effect	(76)	93
Net liabilities at end of period	106,691	102,988
Reinsurance recoverable on unpaid losses at end of period	4,791	4,969
Gross liabilities at end of period	$ 111,482	$ 107,957