Goodwill and Other Intangible Assets (Intangible Assets) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets, Net [Abstract]
Gross Balance		$ 847	$ 728
Accumulated Amortization		(457)	(441)
Net Balance		390	287
Indefinite-lived Intangible Assets (Excluding Goodwill)		46	79
Indefinite- lived intangible assets, Amortization		0	0
Intangible Assets, Gross		893	807
Intangible Assets, Net		436	366
Land Use Rights [Member]
Finite-Lived Intangible Assets, Net [Abstract]
Indefinite-lived Intangible Assets (Excluding Goodwill)		21	45
Indefinite- lived intangible assets, Amortization		0	0
Water rights
Finite-Lived Intangible Assets, Net [Abstract]
Indefinite-lived Intangible Assets (Excluding Goodwill)		20	20
Indefinite- lived intangible assets, Amortization		0	0
Other
Finite-Lived Intangible Assets, Net [Abstract]
Indefinite-lived Intangible Assets (Excluding Goodwill)		5	14
Indefinite- lived intangible assets, Amortization		0	0
Computer Software, Intangible Asset [Member]
Finite-Lived Intangible Assets, Net [Abstract]
Gross Balance		467	416
Accumulated Amortization		(344)	(330)
Net Balance		123	86
Contractual payment rights
Finite-Lived Intangible Assets, Net [Abstract]
Gross Balance		57	65
Accumulated Amortization		(44)	(47)
Net Balance	[1]	13	18
Emission allowances [Member]
Finite-Lived Intangible Assets, Net [Abstract]
Gross Balance		15	0
Accumulated Amortization		0	0
Net Balance	[2]	15	0
Project Development Rights [Member]
Finite-Lived Intangible Assets, Net [Abstract]
Gross Balance		93	57
Accumulated Amortization		(1)	(1)
Net Balance		92	56
Contracts
Finite-Lived Intangible Assets, Net [Abstract]
Gross Balance		137	92
Accumulated Amortization		(24)	(21)
Net Balance		113	71
Other
Finite-Lived Intangible Assets, Net [Abstract]
Gross Balance		78	98
Accumulated Amortization		(44)	(42)
Net Balance	[3]	$ 34	$ 56

[1] Represent legal rights to receive system reliability payments from the regulator.

[2] Acquired or purchased emissions allowances are finite-lived intangible assets that are expensed when utilized and included in net income for the year.

[3] Includes management rights, sales concessions, renewable energy credits and incentives, and other individually insignificant intangible assets. During the fourth quarter of 2018, the Company recognized an asset impairment of $23 million on gas extraction rights at Nejapa. See Note 20—Asset Impairment Expense for further information.