Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
Winter Storm Uri, Hurricane Ida [Member] | Europe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophe events		$ 1,500
Hurricane Ian [Member] | Australia and South Africa [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophe events	$ 3,400
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, prior accident years	(2,141)	(2,151)
Incurred losses and loss adjustment expenses, current accident year	$ 44,472	$ 38,447
Prior years' losses incurred as a percentage of net liabilities	2.60%	2.80%
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Primary Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	$ (734)	$ (1,600)
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Property and Casualty Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	(1,400)	(564)
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Significant catastrophe Events [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year	$ 4,000	2,700
Minimum losses per event each accident year		$ 100