Investment gains/losses (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Gain (Loss) on Investments [Line Items]
Investment gains/losses	$ 315	$ 1,850
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains from sales, redemptions and other disposals	11	19
Gross losses from sales, redemptions and other disposals	(6)	(3)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains from sales, redemptions and other disposals	425	1,807
Gross losses from sales, redemptions and other disposals	(125)	(10)
Other Types of Investments [Member]
Gain (Loss) on Investments [Line Items]
Other investment gains/losses	$ 10	$ 37