Retroactive reinsurance contracts - Narrative (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Effects of Reinsurance [Line Items]
Insurance losses and loss adjustment expenses	$ 1,484	$ 855	$ 981
Increase in incurred losses and loss adjustment expenses	650
Unamortized deferred charges of retroactive reinsurance contracts	9,500	9,900
Liabilities for environmental, asbestos and latent injury claims	12,200	12,100
Unpaid Losses and Loss Adjustment Expenses [Member]
Effects of Reinsurance [Line Items]
Insurance losses and loss adjustment expenses	$ 1,109	$ 86	$ (821)