Investments (Net Realized Gains (Losses) From Sales Of Available-For-Sale Securities) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Proceeds from sales	$ 2,256	$ 2,412	$ 1,145
Available For Sale Securities Realized Gain Loss [Abstract]
Gross realized gains	40	84	32
Gross realized losses	$ (22)	$ (23)	$ (16)