Goodwill and Other Intangible Assets (Intangible Assets Acquired) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Indefinite-lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 112	$ 70
Project Development Rights [Member]
Indefinite-lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 53
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	30 years
Other Intangible Assets [Member]
Indefinite-lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired		$ 5
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		13 years
Other intangible assets acquired	$ 8
Contracts
Indefinite-lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 34	$ 24
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	25 years	26 years
Computer Software, Intangible Asset [Member]
Indefinite-lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired	$ 17	$ 41
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years	4 years