Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
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, current accident year	$ 59,244,000,000	$ 59,463,000,000	$ 52,099,000,000
Incurred losses and loss adjustment expenses, prior accident years	$ (3,541,000,000)	$ (2,672,000,000)	$ (3,116,000,000)
Prior years' losses incurred as a percentage of net liabilities	3.50%	3.20%	4.00%
Liabilities for environmental, asbestos and latent injury exposures	$ 2,000,000,000	$ 2,100,000,000
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year	925,000,000	3,100,000,000	$ 2,900,000,000
Minimum losses per event each accident year	150,000,000
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Hurricane Ian [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year		2,500,000,000
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Hurricane Ida [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses, current accident year			1,500,000,000
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	(2,100,000,000)	(1,100,000,000)	(2,400,000,000)
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	$ (1,400,000,000)	$ (1,600,000,000)	$ (718,000,000)