Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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	$ 162	$ (295)	$ (469)
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	43,400	43,335	39,876
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events	$ (356)	$ (752)	$ (1,406)
Prior years' losses incurred as a percentage of net liabilities	0.50%	1.10%	2.40%
Liabilities included amounts for environmental, asbestos and latent injury claims, net of reinsurance	$ 2,100	$ 1,700
Property and Casualty Insurance and Reinsurance, Excluding Retroactive Reinsurance [Member] | Significant Catastrophe Events [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses with respect to current accident year events	950	1,000	$ 1,600
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	$ (518)	$ (457)	$ (937)