Employee Benefit Plans (Net Periodic Costs (Components of Net Periodic Benefit Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 122	$ 115	$ 116
Interest cost	160	186	171
Expected return on plan assets	(302)	(288)	(306)
Amortization of prior service cost/(credit)	4	4	1
Amortization of actuarial (gain)/loss		4
Net periodic cost (benefit)	(16)	21	(18)
Current year prior service cost/(credit)		21
Amortization of prior service (cost)/credit	(4)	(4)	(1)
Current year actuarial loss/(gain)	(112)	(193)	75
Amortization of actuarial gain/(loss)		(4)
Total recognized in OCI	(116)	(180)	74
Total amounts recognized in net periodic cost and OCI	(132)	(159)	56
Pension Plans | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3	3
Interest cost	8	10	9
Amortization of actuarial (gain)/loss	5	4	5
Net periodic cost (benefit)	16	17	17
Current year actuarial loss/(gain)	17	25	(16)
Amortization of actuarial gain/(loss)	(5)	(4)	(5)
Total recognized in OCI	12	21	(21)
Total amounts recognized in net periodic cost and OCI	28	38	(4)
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	4	5
Interest cost	11	13	12
Expected return on plan assets	(6)	(5)	(6)
Net periodic cost (benefit)	9	12	11
Current year actuarial loss/(gain)	3	9	(25)
Total recognized in OCI	3	9	(25)
Total amounts recognized in net periodic cost and OCI	$ 12	$ 21	$ (14)