Unpaid losses and loss adjustment expenses - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
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	$ (28)	$ 212
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	(172)	(100)
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	$ (200)	$ 112
Prior years' losses incurred as a percentage of net liabilities	0.30%	0.20%