Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property and Casualty Reinsurance, Excluding Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	$ (718)	$ 162	$ (295)
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 with respect to current accident year events	52,099	43,400	43,335
Incurred losses and loss adjustment expenses, prior accident years	$ (3,116)	$ (356)	$ (752)
Prior years' losses incurred as a percentage of net liabilities	4.00%	0.50%	1.10%
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 with respect to current accident year events	2,900	950	$ 1,000
Minimum losses per event each accident year	100
Primary Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	$ (2,400)	$ (518)	$ (457)