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	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 86,664	$ 79,854	$ 73,019
Reinsurance recoverable on unpaid losses at beginning of year	(2,960)	(2,912)	(2,855)
Net liabilities at beginning of year	83,704	76,942	70,164
Incurred losses and loss adjustment expenses, current accident year	59,463	52,099	43,400
Incurred losses and loss adjustment expenses, prior accident years	(2,672)	(3,116)	(356)
Total incurred losses and loss adjustment expenses	56,791	48,983	43,044
Paid losses and loss adjustment expenses, current accident year	(27,236)	(22,897)	(17,884)
Paid losses and loss adjustment expenses, prior accident years	(23,083)	(18,904)	(18,862)
Total paid losses and loss adjustment expenses	(50,319)	(41,801)	(36,746)
Foreign currency effect	(508)	(420)	480
Net liabilities of acquired businesses	12,779
Net liabilities at end of year	102,447	83,704	76,942
Reinsurance recoverable on unpaid losses at end of year	5,025	2,960	2,912
Gross liabilities at end of year	$ 107,472	$ 86,664	$ 79,854