Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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 for (reductions of) prior years' accident events	$ (752)	$ (1,406)	$ (544)
Incurred losses and loss adjustment expenses with respect to current accident year events	$ 43,335	$ 39,876	$ 37,702
Prior years' losses incurred as a percentage of net liabilities	1.10%	2.40%	1.10%
Liabilities included amounts for environmental, asbestos and latent injury claims, net of reinsurance	$ 1,700	$ 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	1,000	1,600	$ 3,000
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	(457)	(937)	(249)
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	$ (295)	$ (469)	$ (295)