Unpaid losses and loss adjustment expenses - Incurred losses for prior accident years (Detail) - Property/Casualty [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Unpaid Losses and Loss Adjustment Expenses [Line Items]
Incurred losses before the effects of deferred charges and liability discounts	$ (1,545)	$ (1,493)	$ (1,938)
Incurred losses from changes in deferred charges and liability discounts	243	128	186
Incurred losses prior years	$ (1,302)	$ (1,365)	$ (1,752)