Employee Benefit Plans (Reconciliation of Changes in Projected Benefit Obligation and Change in Plan Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, beginning balance	$ 5,654
Fair value of plan assets, end balance	6,073	$ 5,654
Pension Plans | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	5,117	4,740
Projected benefit obligation at beginning of year	4,887	4,355
Service cost	122	115	$ 116
Interest cost	160	186	171
Amendments		21
Actuarial (gains)/losses and changes in assumptions	306	498
Benefits paid	(301)	(288)
Projected benefit obligation at end of year	5,174	4,887	4,355
Fair value of plan assets, beginning balance	5,654	4,963
Actual return on plan assets	720	979
Benefits paid	(301)	(288)
Fair value of plan assets, end balance	6,073	5,654	4,963
Funded status	899	767
Noncurrent asset	899	767
Net amount recognized on the consolidated balance sheet	899	767
Prior service cost (credit)	25	29
Net actuarial loss (gain)	299	411
Amount recognized in AOCI	324	440
Pension Plans | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	258	259
Projected benefit obligation at beginning of year	265	258
Service cost	3	3	3
Interest cost	8	10	9
Actuarial (gains)/losses and changes in assumptions	17	25
Benefits paid	(30)	(31)
Projected benefit obligation at end of year	263	265	258
Fair value of plan assets, beginning balance
Employer contribution	30	31
Benefits paid	(30)	(31)
Fair value of plan assets, end balance
Funded status	(263)	(265)
Current liability	(25)	(26)
Noncurrent liability	(238)	(239)
Net amount recognized on the consolidated balance sheet	(263)	(265)
Net actuarial loss (gain)	90	78
Amount recognized in AOCI	90	78
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation at beginning of year	333	322
Service cost	4	4	5
Interest cost	11	13	12
Actuarial (gains)/losses and changes in assumptions	11	18
Participant contributions	3	3
Benefits paid	(25)	(27)
Projected benefit obligation at end of year	337	333	322
Fair value of plan assets, beginning balance	247	232
Actual return on plan assets	14	15
Employer contribution	23	24
Participant contributions	3	3
Benefits paid	(25)	(27)
Fair value of plan assets, end balance	262	247	$ 232
Funded status	(75)	(86)
Current liability	(2)	(2)
Noncurrent liability	(73)	(84)
Net amount recognized on the consolidated balance sheet	(75)	(86)
Prior service cost (credit)	2	2
Net actuarial loss (gain)	4	1
Amount recognized in AOCI	$ 6	$ 3