Unpaid losses and loss adjustment expenses - Narrative (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Event	Dec. 31, 2017 USD ($) Event	Dec. 31, 2016 USD ($)
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	$ 39,876	$ 37,702	$ 30,636
Number of significant catastrophe events | Event	4	6
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events	$ (1,406)	$ (544)	$ (1,443)
Percentage of reduction of net liabilities	2.40%	1.10%	3.10%
Liabilities for environmental, asbestos and latent injury claims, net of reinsurance	$ 1,700	$ 1,600
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	1,600	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	(937)	(249)	$ (569)
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	$ (469)	$ (295)	$ (874)