Unpaid losses and loss adjustment expenses - Narrative (Detail) - Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] - USD ($)	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred, current accident year	$ 14,638,000,000	$ 13,854,000,000
Losses and loss adjustment expenses incurred, prior accident years	$ (163,000,000)	$ (634,000,000)
Prior years' losses incurred as a percentage of net liabilities	0.10%	0.60%
Primary Insurance Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred, prior accident years	$ 167,000,000	$ (248,000,000)
Property and Casualty Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Losses and loss adjustment expenses incurred, prior accident years	(330,000,000)	(386,000,000)
Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Minimum losses per event each accident year	150,000,000
Losses and loss adjustment expenses incurred, current accident year	$ 1,100,000,000	$ 0