Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred, current accident year	$ 57,563,000,000	$ 59,244,000,000	$ 59,463,000,000
Losses and loss adjustment expenses incurred, prior accident years	$ (2,322,000,000)	$ (3,541,000,000)	$ (2,672,000,000)
Prior years' losses incurred as a percentage of net liabilities	2.20%	3.50%	3.20%
Liabilities for environmental, asbestos and latent injury exposures	$ 1,900,000,000	$ 2,000,000,000
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred, current accident year	1,500,000,000	925,000,000	$ 3,100,000,000
Minimum losses per event each accident year	150,000,000
Property and Casualty Reinsurance, Excluding Retroactive Reinsurance [Member] | Primary Insurance Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred, prior accident years	(602,000,000)	(2,100,000,000)	(1,100,000,000)
Property and Casualty Reinsurance, Excluding Retroactive Reinsurance [Member] | Property And Casualty Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred, prior accident years	$ (1,700,000,000)	$ (1,400,000,000)	$ (1,600,000,000)