Revenue Recognition (Disaggregation of Revenue) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating revenue from contracts with customers	$ 2	$ 2	$ 3	$ 3
Total operating revenues	698	632	363	1,334
Residential [Member] | Electric Operations
Operating revenue from contracts with customers	281	243	10	495
Residential [Member] | Gas Distribution
Operating revenue from contracts with customers	31	34	109	120
Commercial [Member] | Electric Operations
Operating revenue from contracts with customers	206	171	67	340
Commercial [Member] | Gas Distribution
Operating revenue from contracts with customers	23	22	61	61
Industrial [Member] | Electric Operations
Operating revenue from contracts with customers	100	106	18	191
Industrial [Member] | Gas Distribution
Operating revenue from contracts with customers	19	21	45	45
Other | Electric Operations
Operating revenue from contracts with customers	35	30	45	68
Other | Gas Distribution
Operating revenue from contracts with customers	4	2	7	7
Electric Operations
Operating revenue from contracts with customers	622	550	140	1,094
Other revenues	(1)	3	1	6
Total operating revenues	621	553	141	1,100
Gas Distribution
Operating revenue from contracts with customers	77	79	222	233
Other revenues	0	0	0	1
Total operating revenues	$ 77	$ 79	$ 222	$ 234