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, 2022	Mar. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 86,664	$ 79,854
Reinsurance recoverable on unpaid losses at beginning of year	(2,960)	(2,912)
Net liabilities at beginning of year	83,704	76,942
Incurred losses and loss adjustment expenses, current accident year	13,375	11,363
Incurred losses and loss adjustment expenses, prior accident years	(251)	(646)
Total incurred losses and loss adjustment expenses	13,124	10,717
Paid losses and loss adjustment expenses, current accident year	(3,908)	(3,214)
Paid losses and loss adjustment expenses, prior accident years	(8,091)	(6,454)
Total paid losses and loss adjustment expenses	(11,999)	(9,668)
Foreign currency effect	(106)	(94)
Net liabilities at end of period	84,723	77,897
Reinsurance recoverable on unpaid losses at end of period	2,944	3,064
Gross liabilities at end of period	$ 87,667	$ 80,961