Investment gains/losses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses		$ (26)	$ (63)	$ (26)
Investment gains/losses	$ 3,150	8,266	5,643	8,725
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains from sales and redemptions	5	6	44	88
Gross losses from sales and redemptions	(24)	(44)	(41)	(128)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains from sales and redemptions	3,173	8,407	5,720	8,855
Gross losses from sales and redemptions	(13)	(75)	(76)	(95)
Other Types of Investments [Member]
Gain (Loss) on Investments [Line Items]
Other investment gains/losses	$ 9	$ (2)	$ 59	$ 31