Employee Benefit Plans (Reconciliation of Changes in Projected Benefit Obligation and Change in Plan Assets) (Details) - USD ($)	1 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Qualified Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year		$ 4,315,000,000	$ 4,726,000,000
Projected benefit obligation at beginning of year		4,789,000,000	4,547,000,000
Service cost		116,000,000	160,000,000	$ 102,000,000
Interest cost		171,000,000	179,000,000	186,000,000
Amendments			17,000,000
Actuarial (gains)/losses and changes in assumptions		(424,000,000)	172,000,000
Benefits paid		(297,000,000)	(286,000,000)
Projected benefit obligation at end of year		4,355,000,000	4,789,000,000	4,547,000,000
Fair value of plan assets, beginning balance		5,253,000,000	4,617,000,000
Actual return on plan assets		(193,000,000)	722,000,000
Employer contribution	$ 200,000,000	200,000,000	200,000,000
Benefits paid		(297,000,000)	(286,000,000)
Fair value of plan assets, end balance		4,963,000,000	5,253,000,000	4,617,000,000
Funded status		608,000,000	464,000,000
Noncurrent asset		608,000,000	464,000,000
Net amount recognized on the consolidated balance sheet		608,000,000	464,000,000
Prior service cost (credit)		12,000,000	13,000,000
Net actuarial loss (gain)		608,000,000	534,000,000
Amount recognized in AOCI		620,000,000	547,000,000
Nonqualified Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year		253,000,000	280,000,000
Projected benefit obligation at beginning of year		286,000,000	289,000,000
Service cost		3,000,000	3,000,000	3,000,000
Interest cost		9,000,000	10,000,000	12,000,000
Actuarial (gains)/losses and changes in assumptions		(16,000,000)	8,000,000
Benefits paid		(24,000,000)	(24,000,000)
Projected benefit obligation at end of year		258,000,000	286,000,000	289,000,000
Fair value of plan assets, beginning balance		0
Employer contribution		24,000,000	24,000,000
Benefits paid		(24,000,000)	(24,000,000)
Fair value of plan assets, end balance		0	0
Funded status		(258,000,000)	(286,000,000)
Current liability		(26,000,000)	(28,000,000)
Noncurrent liability		(232,000,000)	(258,000,000)
Net amount recognized on the consolidated balance sheet		(258,000,000)	(286,000,000)
Net actuarial loss (gain)		57,000,000	77,000,000
Amount recognized in AOCI		57,000,000	77,000,000
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation at beginning of year		355,000,000	373,000,000
Service cost		5,000,000	5,000,000	6,000,000
Interest cost		12,000,000	14,000,000	15,000,000
Amendments			2,000,000
Actuarial (gains)/losses and changes in assumptions		(28,000,000)	(18,000,000)
Participant contributions		3,000,000	3,000,000
Federal Medicare subsidy on benefits paid		1,000,000	1,000,000
Benefits paid		(26,000,000)	(25,000,000)
Projected benefit obligation at end of year		322,000,000	355,000,000	373,000,000
Fair value of plan assets, beginning balance		230,000,000	208,000,000
Actual return on plan assets		3,000,000	9,000,000
Employer contribution		21,000,000	34,000,000
Participant contributions		3,000,000	3,000,000
Federal Medicare subsidy on benefits paid		1,000,000	1,000,000
Benefits paid		(26,000,000)	(25,000,000)
Fair value of plan assets, end balance		232,000,000	230,000,000	$ 208,000,000
Funded status		(90,000,000)	(125,000,000)
Current liability		(2,000,000)	(2,000,000)
Noncurrent liability		(88,000,000)	(123,000,000)
Net amount recognized on the consolidated balance sheet		(90,000,000)	(125,000,000)
Prior service cost (credit)		1,000,000	1,000,000
Net actuarial loss (gain)		(7,000,000)	18,000,000
Amount recognized in AOCI		$ (6,000,000)	$ 19,000,000