Unpaid losses and loss adjustment expenses - Reconciliation of changes in claim liabilities (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross liabilities at beginning of year	$ 79,854	$ 73,019
Reinsurance recoverable on unpaid losses at beginning of year	(2,912)	(2,855)
Net liabilities at beginning of year	76,942	70,164
Net liabilities at end of period	82,766	75,184
Reinsurance recoverable on unpaid losses at end of period	2,988	2,857
Gross liabilities at end of period	85,754	78,041
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year events	38,447	31,777
Incurred losses and loss adjustment expenses, prior accident years' events	(2,151)	69
Total	36,296	31,846
Paid losses and loss adjustment expenses, current accident year events	(15,277)	(11,691)
Paid losses and loss adjustment expenses, prior accident years' events	(15,076)	(15,259)
Total	(30,353)	(26,950)
Foreign currency translation adjustment	$ (119)	$ 124