Investment gains/losses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairment losses				$ (63)
Investment gains/losses	$ 657	$ 3,150	$ 1,262	5,643
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains	56	5	82	44
Gross losses	(2)	(24)	(16)	(41)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains	1,011	3,173	1,795	5,720
Gross losses	(419)	(13)	(626)	(76)
Other Types of Investments [Member]
Gain (Loss) on Investments [Line Items]
Other investment gains/losses	$ 11	$ 9	$ 27	$ 59