Employee Benefit Plans (Accumulated Other Comprehensive Loss Pretax Amounts Not Yet Reflected in Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated net actuarial losses	$ 1,071	$ 920
Accumulated prior service cost (credit)	4	2
Total	1,075	922
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated net actuarial losses	233	166
Accumulated prior service cost (credit)	(75)	(138)
Total	$ 158	$ 28