Investments (Net Realized Gains (Losses) From Sales Of Available-For-Sale Securities) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments [Abstract]
Proceeds from sales	$ 583	$ 312	$ 1,647	$ 1,300
Available For Sale Securities Realized Gain Loss [Abstract]
Gross realized gains	1	5	4	24
Gross realized losses	$ (2)	$ (5)	$ (15)	$ (9)