Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Hurricane Ian [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophe events	$ 2,500
Europe [Member] | Winter Storm Uri, Hurricane Ida [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophe events		$ 1,500
United States [Member] | Hurricanes Laura and Sally [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss from catastrophe events			$ 600
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	59,463	52,099	43,400
Incurred losses and loss adjustment expenses, prior accident years	$ (2,672)	$ (3,116)	$ (356)
Prior years' losses incurred as a percentage of net liabilities	3.20%	4.00%	0.50%
Liabilities for environmental, asbestos and latent injury exposures	$ 2,100	$ 2,100
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	3,100	2,900	$ 950
Minimum losses per event each accident year	150
Property and Casualty Reinsurance, Excluding Retroactive Reinsurance [Member] | Primary Insurance Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	(1,100)	(2,400)	(518)
Property and Casualty 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	$ (1,600)	$ (718)	$ 162