Segment and Related Information (Geographic Data) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 1,939	$ 1,872	$ 1,848	$ 2,126	$ 1,926	$ 1,845	$ 1,725	$ 1,951	$ 7,785	$ 7,447	$ 7,517
Long-lived assets	35,175				33,787				35,175	33,787
U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									5,517	5,356	5,310
Long-lived assets	22,384				20,936				22,384	20,936
U.K. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 12,791				$ 12,851				12,791	12,851
U.K. Regulated [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									$ 2,268	$ 2,091	$ 2,207