Retroactive reinsurance contracts - Narrative (Detail) - Retroactive Reinsurance [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Effects of Reinsurance [Line Items]
Insurance losses and loss adjustment expenses	$ 945	$ 1,484	$ 855
Increase in incurred losses and loss adjustment expenses		650
Deferred charge assets on retroactive reinsurance contracts	8,800	9,500
Liabilities for environmental, asbestos and latent injury claims	11,900	12,200
Unpaid Losses and Loss Adjustment Expenses [Member]
Effects of Reinsurance [Line Items]
Insurance losses and loss adjustment expenses	$ 247	$ 1,109	$ 86