Employee Benefit Plans (Summary of Changes in the Corporation's Qualified Defined Benefit Pension Plan's Level 3 Investments Measured at Fair Value on a Recurring Basis) (Details) - Private placements - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Balance at beginning of period	$ 60	$ 80
Gains (Losses) Realized	3	(1)
Gains (Losses) Unrealized	8	(7)
Purchases	49	70
Sales	(63)	(82)
Balance at end of period	$ 57	$ 60