Investment Securities (Summary Of Continuous Unrealized Loss Positions Of Available-For-Sale Security) (Details) - Accumulated Other Comprehensive Income (Loss) [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Unrealized Loss Position For - Estimated Fair Value - Less than 12 Months	$ 7	$ 1,653
Unrealized Loss Position For - Unrealized Loss - Less than 12 Months	(1)	(12)
Unrealized Loss Position For - Estimated Fair Value - 12 Months or More	231	6,984
Unrealized Loss Position For - Unrealized Loss - 12 Months or More	(2)	(63)
Unrealized Loss Position For - Estimated Fair Value - Total	238	8,637
Unrealized Loss Position For - Unrealized Loss - Total	$ (3)	$ (75)