Investments (Portion Of Unrealized Gains And Losses On Equity Investments Held) (Detail) - Equity Securities [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Gains (losses) on equity investments [Member]
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 5	$ 0
Gains (losses) on equity investments sold during the period [Member]
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	1	0
Gains (losses) on equity investments still held at the end of the period [Member]
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	$ 4	$ 0