Investment gains/losses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Investment gains/losses	$ 1,410	$ 7,553	$ 9,373
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains	103	58	104
Gross losses	(22)	(51)	(171)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross gains	2,237	7,853	9,526
Gross losses	(919)	(415)	(129)
Other Types of Investments [Member]
Gain (Loss) on Investments [Line Items]
Investment gains/losses	$ 11	$ 108	$ 43