Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
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	$ (740)	$ (251)
Prior years' losses incurred as a percentage of net liabilities	0.70%	0.30%
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	$ (379)	$ (114)
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	$ (361)	$ (137)