Investment gains/losses (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses		$ (63)		$ (63)
Investment gains/losses	$ 290	643	$ 605	2,493
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains from sales and redemptions	15	20	26	39
Gross losses from sales and redemptions	(8)	(14)	(14)	(17)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains from sales and redemptions	359	740	784	2,547
Gross losses from sales and redemptions	(82)	(53)	(207)	(63)
Other Types of Investments [Member]
Gain (Loss) on Investments [Line Items]
Other investment gains/losses	$ 6	$ 13	$ 16	$ 50