Investments (Net Realized Gains (Losses) From Sales Of Available-For-Sale Securities) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments [Abstract]
Proceeds from sales	$ 312	$ 848	$ 1,300	$ 1,785
Available For Sale Securities Realized Gain Loss [Abstract]
Gross realized gains	5	33	24	70
Gross realized losses	$ (5)	$ 0	$ (9)	$ (18)