Segment and Related Information (Geographic Data) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 1,954	$ 1,933	$ 1,803	$ 2,079	$ 1,939	$ 1,872	$ 1,848	$ 2,126	$ 7,769	$ 7,785	$ 7,447
Long-lived assets	37,225				35,175				37,225	35,175
U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									5,602	5,517	5,356
Long-lived assets	23,607				22,384				23,607	22,384
U.K. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 13,618				$ 12,791				13,618	12,791
U.K. Regulated [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									$ 2,167	$ 2,268	$ 2,091