Segment Information (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020		Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	$ 1,744	[1]	$ 2,599	[1]	$ 4,036	[2]	$ 5,198	[2]
Commodity Margin	723		752		1,311		1,531
Add: Mark-to-market commodity activity, net and other(2)	59	[3]	200	[3]	272	[4]	252	[4]
Operating and maintenance expense	266		245		506		484
Depreciation and amortization expense	163		175		327		349
General and other administrative expense	31		34		62		66
Other operating expenses	14		19		31		38
(Income) loss from unconsolidated investments in power plants	(4)		(5)		(4)		(11)
Income from operations	312		444		661		802
Interest expense	167		157		336		306
Debt Extinguishment Costs and Other (Income) Expense, Net	13		8		17		27
Income before income taxes	132		279		308		469
Lease levelization	(22)		(19)		(40)		(35)
Impairment losses	0		40		0		55
West
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	470	[1]	649	[1]	1,175	[2]	1,331	[2]
Commodity Margin	269		251		503		515
Add: Mark-to-market commodity activity, net and other(2)	67	[3]	58	[3]	121	[4]	114	[4]
Operating and maintenance expense	96		84		182		165
Depreciation and amortization expense	56		60		112		133
General and other administrative expense	6		5		14		12
Other operating expenses	7		7		15		16
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	171		153		301		303
Impairment losses			0				0
Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	620	[1]	899	[1]	1,270	[2]	1,642	[2]
Commodity Margin	172		173		285		335
Add: Mark-to-market commodity activity, net and other(2)	(17)	[3]	240	[3]	84	[4]	284	[4]
Operating and maintenance expense	73		66		139		131
Depreciation and amortization expense	50		54		100		99
General and other administrative expense	13		15		24		27
Other operating expenses	1		1		3		3
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	18		277		103		359
Impairment losses			0				0
East
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	244	[1]	646	[1]	786	[2]	1,335	[2]
Commodity Margin	193		235		343		500
Add: Mark-to-market commodity activity, net and other(2)	(38)	[3]	94	[3]	44	[4]	107	[4]
Operating and maintenance expense	71		72		134		139
Depreciation and amortization expense	45		48		91		91
General and other administrative expense	8		10		16		19
Other operating expenses	6		11		13		19
(Income) loss from unconsolidated investments in power plants	(4)		(6)		(4)		(12)
Income from operations	29		154		137		296
Impairment losses			40				55
Retail
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	720	[1]	1,082	[1]	1,628	[2]	2,080	[2]
Commodity Margin	89		93		180		181
Add: Mark-to-market commodity activity, net and other(2)	56	[3]	(182)	[3]	40	[4]	(235)	[4]
Operating and maintenance expense	35		33		68		67
Depreciation and amortization expense	12		13		24		26
General and other administrative expense	4		4		8		8
Other operating expenses	0		0		0		0
(Income) loss from unconsolidated investments in power plants	0		1		0		1
Income from operations	94		(140)		120		(156)
Impairment losses			0				0
Consolidation, Eliminations
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	(310)	[1]	(677)	[1]	(823)	[2]	(1,190)	[2]
Commodity Margin	0		0		0		0
Add: Mark-to-market commodity activity, net and other(2)	(9)	[3]	(10)	[3]	(17)	[4]	(18)	[4]
Operating and maintenance expense	(9)		(10)		(17)		(18)
Depreciation and amortization expense	0		0		0		0
General and other administrative expense	0		0		0		0
Other operating expenses	0		0		0		0
(Income) loss from unconsolidated investments in power plants	0		0		0		0
Income from operations	0		0		0		0
Impairment losses			0				0
Elimination | West
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	70		100		189		262
Elimination | Texas
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	214		348		432		559
Elimination | East
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	24		228		199		365
Elimination | Retail
Revenues from External Customers and Long-Lived Assets [Line Items]
Total operating revenues	2		1		3		4
Other Assets
Revenues from External Customers and Long-Lived Assets [Line Items]
Amortization of Intangible Assets	$ 9		$ 18		$ 25		$ 39

[1] Includes intersegment revenues of $70 million and $100 million in the West, $214 million and $348 million in Texas, $24 million and $228 million in the East and $2 million and $1 million in Retail for the three months ended June 30, 2020 and 2019, respectively.

[2] Includes intersegment revenues of $189 million and $262 million in the West, $432 million and $559 million in Texas, $199 million and $365 million in the East and $3 million and $4 million in Retail for the six months ended June 30, 2020 and 2019, respectively.

[3] Includes $(22) million and $(19) million of lease levelization and $9 million and $18 million of amortization expense for the three months ended June 30, 2020 and 2019, respectively.

[4] Includes $(40) million and $(35) million of lease levelization and $25 million and $39 million of amortization expense for the six months ended June 30, 2020 and 2019, respectively.