Benefit Plans (Fair Values of Pension Plan Assets) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	$ 8,015	$ 7,475
U.S. And Foreign Government Securities Or Their Agencies [Member] | U.S. government and agencies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	588	444
U.S. And Foreign Government Securities Or Their Agencies [Member] | U.S. municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	81	102
U.S. And Foreign Government Securities Or Their Agencies [Member] | Foreign government and agencies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	150	185
Corporate Debt Instruments [Member] | Above investment grade [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	1,789	1,735
Corporate Debt Instruments [Member] | Below grade investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	511	602
Common Stock [Member] | International equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	1,396	1,076
Common Stock [Member] | U.S. equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	831	760
Other, Net [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	194	188
Plan Asset Categories, Excluding Investments Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	5,540	5,092
Common/Collective Trusts [Member] | U.S. large cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	2,014	1,940
Common/Collective Trusts [Member] | U.S. small cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	361	363
Common/Collective Trusts [Member] | International developed markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	100	80
Level 1 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | U.S. government and agencies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 1 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | U.S. municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 1 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | Foreign government and agencies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 1 [Member] | Corporate Debt Instruments [Member] | Above investment grade [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 1 [Member] | Corporate Debt Instruments [Member] | Below grade investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 1 [Member] | Common Stock [Member] | International equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	1,396	1,076
Level 1 [Member] | Common Stock [Member] | U.S. equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	831	760
Level 1 [Member] | Other, Net [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	120	142
Level 1 [Member] | Plan Asset Categories, Excluding Investments Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	2,347	1,978
Level 2 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | U.S. government and agencies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	588	444
Level 2 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | U.S. municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	81	102
Level 2 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | Foreign government and agencies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	150	185
Level 2 [Member] | Corporate Debt Instruments [Member] | Above investment grade [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	1,789	1,735
Level 2 [Member] | Corporate Debt Instruments [Member] | Below grade investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	511	602
Level 2 [Member] | Common Stock [Member] | International equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 2 [Member] | Common Stock [Member] | U.S. equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 2 [Member] | Other, Net [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	74	33
Level 2 [Member] | Plan Asset Categories, Excluding Investments Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	3,193	3,101
Level 3 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | U.S. government and agencies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 3 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | U.S. municipal bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 3 [Member] | U.S. And Foreign Government Securities Or Their Agencies [Member] | Foreign government and agencies [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 3 [Member] | Corporate Debt Instruments [Member] | Above investment grade [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 3 [Member] | Corporate Debt Instruments [Member] | Below grade investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 3 [Member] | Common Stock [Member] | International equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 3 [Member] | Common Stock [Member] | U.S. equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	0
Level 3 [Member] | Other, Net [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	0	13
Level 3 [Member] | Plan Asset Categories, Excluding Investments Measured at NAV [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, net	$ 0	$ 13