Operating Segments (Narrative) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Charge for refund of amounts from customers	$ 71	$ 73
Operating Segments
Segment Reporting Information [Line Items]
After- tax net expenses	1,400
Merger and integration related costs	90
Merger and integration-related costs, after tax	68
Charge related to a voluntary retirement program	75
Charge related to a voluntary retirement program, after-tax	56
Operating Segments | Corporate and Other
Segment Reporting Information [Line Items]
After- tax net expenses	1,400
Operating Segments | Electric Operations
Segment Reporting Information [Line Items]
Charge for refund of amounts from customers	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
Income tax related to regulatory assets acquired, after tax	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	61
Changes in unrecognized tax benefits	59
Operating Segments | Gas Distribution
Segment Reporting Information [Line Items]
Merger and integration-related costs, after tax	$ 7