Operating Segments (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Charge for refund of amounts from customers	$ 75	$ 75	$ 73
Litigation settlement expense	100	266
Litigation settlement expense, after tax	75	200
Operating Segments
Segment Reporting Information [Line Items]
After- tax net expenses		1,400
Merger and integration related costs		72
Merger and integration-related costs, after tax		60
Charge related to a voluntary retirement program		62
Charge related to a voluntary retirement program, after-tax		47
Operating Segments | Corporate and Other
Segment Reporting Information [Line Items]
After- tax net expenses		1,300
Operating Segments | Electric Operations
Segment Reporting Information [Line Items]
Charge for refund of amounts from customers	1,000	1,000
Charge for refund of amounts from customers, after tax		756
Litigation settlement expense		266
Litigation settlement expense, after tax		200
Income tax related to regulatory assets acquired	264	264
Income tax related to regulatory assets acquired, after tax	$ 198	198
Charge for utility plant but committed to forgo recovery		114
Charge for utility plant but committed to forgo recovery, after tax		86
Merger and integration-related costs, after tax		55
Changes in unrecognized tax benefits		63
Operating Segments | Gas Distribution
Segment Reporting Information [Line Items]
Merger and integration-related costs, after tax		$ 5