Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 37,702	$ 30,636	$ 27,829
Incurred losses and loss adjustment expenses for (reductions of) prior years' accident events	(544)	(1,523)	(2,015)
Liabilities for environmental, asbestos and latent injury claims, net of reinsurance	1,600	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	(249)	(569)	(793)
Hurricanes Harvey, Irma and Maria, an Earthquake in Mexico, a Cyclone in Australia and Wildfires in California [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Incurred losses and loss adjustment expenses with respect to current accident year events	3,000
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)	$ (955)	$ (1,200)