Operating Segments (Schedule of Segment Reporting Information, by Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
External revenue	$ 698	$ 632	$ 363	$ 1,334
Comprehensive income (loss) available (attributable) to common shareholder	(78)	26	(1,187)	150
Adjustments/Eliminations
Segment Reporting Information [Line Items]
External revenue	0	0	0	0
Comprehensive income (loss) available (attributable) to common shareholder	(8)	(5)	(14)	(9)
Electric Operations
Segment Reporting Information [Line Items]
External revenue	621	553	141	1,100
Electric Operations | Operating Segments
Segment Reporting Information [Line Items]
External revenue	620	553	1,147	1,100
Comprehensive income (loss) available (attributable) to common shareholder	104	30	152	130
Gas Distribution
Segment Reporting Information [Line Items]
External revenue	77	79	222	234
Gas Distribution | Operating Segments
Segment Reporting Information [Line Items]
External revenue	78	79	225	234
Comprehensive income (loss) available (attributable) to common shareholder	(8)	(3)	14	29
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
External revenue	0	0	(1,009)	0
Comprehensive income (loss) available (attributable) to common shareholder	$ (166)	$ 4	$ (1,339)	$ 0