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, 2023	Mar. 31, 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	14,776	13,375
Incurred losses and loss adjustment expenses, prior accident years	(740)	(251)
Total incurred losses and loss adjustment expenses	14,036	13,124
Paid losses and loss adjustment expenses, current accident year	(3,841)	(3,908)
Paid losses and loss adjustment expenses, prior accident years	(9,747)	(8,091)
Total paid losses and loss adjustment expenses	(13,588)	(11,999)
Foreign currency effect	93	(106)
Net liabilities at end of period	102,988	84,723
Reinsurance recoverable on unpaid losses at end of period	4,969	2,944
Gross liabilities at end of period	$ 107,957	$ 87,667