Segment and Related Information (Geographic Data) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 7,607	$ 7,769	$ 7,785
Long-lived assets	39,656	37,225
U.S. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	5,474	5,602	5,517
Long-lived assets	24,851	23,607
U.K. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	14,805	13,618
U.K. Regulated [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 2,133	$ 2,167	$ 2,268