Segment and Related Information (Geographic Data) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 1,926	$ 1,845	$ 1,725	$ 1,951	$ 1,832	$ 1,889	$ 1,785	$ 2,011	$ 7,447	$ 7,517	$ 7,669
Long-lived assets	33,787				30,772				33,787	30,772
U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									5,356	5,310	5,259
Long-lived assets	20,936				19,595				20,936	19,595
U.K. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 12,851				$ 11,177				12,851	11,177
U.K. Regulated [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									$ 2,091	$ 2,207	$ 2,410