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, 2025	Mar. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at the beginning of the year	$ 115,151	$ 111,082
Reinsurance recoverable on unpaid losses at the beginning of the year	(4,593)	(4,893)
Net liabilities at the beginning of the year	110,558	106,189
Losses and loss adjustment expenses incurred, current accident year	14,638	13,854
Losses and loss adjustment expenses incurred, prior accident years	(163)	(634)
Total losses and loss adjustment expenses incurred	14,475	13,220
Losses and loss adjustment expenses paid, current accident year	(3,788)	(3,663)
Losses and loss adjustment expenses paid, prior accident years	(9,105)	(8,979)
Total losses and loss adjustment expenses paid	(12,893)	(12,642)
Foreign currency effect	198	(76)
Net liabilities at end of period	112,338	106,691
Reinsurance recoverable on unpaid losses at end of period	4,785	4,791
Gross liabilities at end of period	$ 117,123	$ 111,482