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, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 107,472	$ 86,664	$ 79,854
Reinsurance recoverable on unpaid losses at beginning of year	(5,025)	(2,960)	(2,912)
Net liabilities at beginning of year	102,447	83,704	76,942
Incurred losses and loss adjustment expenses, current accident year	59,244	59,463	52,099
Incurred losses and loss adjustment expenses, prior accident years	(3,541)	(2,672)	(3,116)
Total incurred losses and loss adjustment expenses	55,703	56,791	48,983
Paid losses and loss adjustment expenses, current accident year	(25,184)	(27,236)	(22,897)
Paid losses and loss adjustment expenses, prior accident years	(27,065)	(23,083)	(18,904)
Total paid losses and loss adjustment expenses	(52,249)	(50,319)	(41,801)
Foreign currency effect	288	(508)	(420)
Net liabilities of acquired businesses		12,779
Net liabilities at end of year	106,189	102,447	83,704
Reinsurance recoverable on unpaid losses at end of year	4,893	5,025	2,960
Gross liabilities at end of year	$ 111,082	$ 107,472	$ 86,664