Unpaid losses and loss adjustment expenses - Narrative (Detail) - Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	$ (634)	$ (740)
Prior years' losses incurred as a percentage of net liabilities	0.60%	0.70%
Primary Insurance Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	$ (248)	$ (379)
Property and Casualty Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, prior accident years	$ (386)	$ (361)