Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020
Property and Casualty Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events		$ (564,000,000)	$ 342,000,000
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 with respect to current accident year events		38,447,000,000	31,777,000,000
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events		$ (2,151,000,000)	$ 69,000,000
Prior years' losses incurred as a percentage of net liabilities		2.80%	0.10%
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Significant Catastrophe Events [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Minimum losses per significant event in current accident year		$ 100,000,000
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Significant Catastrophe Events [Member] | U.S. and Europe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses with respect to current accident year events	$ 2,200,000,000	2,700,000,000
Primary Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events		$ (1,600,000,000)	$ (273,000,000)