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, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at the beginning of the year	$ 111,082	$ 107,472	$ 86,664
Reinsurance recoverable on unpaid losses at the beginning of the year	(4,893)	(5,025)	(2,960)
Net liabilities at the beginning of the year	106,189	102,447	83,704
Losses and loss adjustment expenses incurred, current accident year	57,563	59,244	59,463
Losses and loss adjustment expenses incurred, prior accident years	(2,322)	(3,541)	(2,672)
Total Losses and loss adjustment expenses incurred	55,241	55,703	56,791
Losses and loss adjustment expenses paid, current accident year	(24,139)	(25,184)	(27,236)
Losses and loss adjustment expenses paid, prior accident years	(26,436)	(27,065)	(23,083)
Total Losses and loss adjustment expenses paid	(50,575)	(52,249)	(50,319)
Foreign currency effect	(297)	288	(508)
Net liabilities of acquired businesses			12,779
Net liabilities at end of year	110,558	106,189	102,447
Reinsurance recoverable on unpaid losses at end of year	4,593	4,893	5,025
Gross liabilities at end of year	$ 115,151	$ 111,082	$ 107,472