Supplemental Guarantor Condensed Financial Information (Condensed Consolidating Statements of Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended			5 Months Ended	9 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,737		$ 1,807		$ 4,265	$ 4,731
Fuel, purchased power costs and delivery fees	(831)		(868)		(2,090)	(2,256)
Net gain from commodity hedging and trading activities	103		75		226	(118)
Operating costs	(189)		(204)		(598)	(660)
Depreciation and amortization	(200)		(327)		(634)	(983)
Selling, general and administrative expenses	(174)		(163)		(495)	(527)
Impairment of goodwill	(700)	$ (700)	0		(1,400)	0
Impairment of long-lived assets	(1,295)		(9)		(1,971)	(30)
Other income	4		5		15	11
Other deductions	(25)		(5)		(86)	(8)
Interest income	0		0		0	0
Interest expense and related charges	(326)		(325)		(967)	(1,434)
Reorganization items	(39)		(45)	$ (468)	(152)	(468)
Loss before income taxes	(1,935)		(59)		(3,887)	(1,742)
Income tax (expense) benefit	417		21		816	574
Equity earnings of subsidiaries	0		0		0	0
Net loss	(1,518)		(38)		(3,071)	(1,168)
Other comprehensive income	0		0		1	1
Comprehensive income (loss)	(1,518)		(38)		(3,070)	(1,167)
Parent Guarantor [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues	0		0		0	0
Fuel, purchased power costs and delivery fees	0		0		0	0
Net gain from commodity hedging and trading activities	0		0		0	0
Operating costs	0		0		0	0
Depreciation and amortization	0		0		0	0
Selling, general and administrative expenses	0		0		0	0
Impairment of goodwill	0				0
Impairment of long-lived assets	0		0		0	0
Other income	0		0		0	0
Other deductions	0		0		0	0
Interest income	0		0		0	1
Interest expense and related charges	(1)		(2)		(4)	(9)
Reorganization items	0		0		0	0
Loss before income taxes	(1)		(2)		(4)	(8)
Income tax (expense) benefit	0		1		0	2
Equity earnings of subsidiaries	(1,517)		(37)		(3,067)	(1,162)
Net loss	(1,518)		(38)		(3,071)	(1,168)
Other comprehensive income	0		0		1	1
Comprehensive income (loss)	(1,518)		(38)		(3,070)	(1,167)
Issuer [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues	0		0		0	0
Fuel, purchased power costs and delivery fees	0		0		0	0
Net gain from commodity hedging and trading activities	0		0		0	(90)
Operating costs	0		0		0	0
Depreciation and amortization	(2)		0		(5)	0
Selling, general and administrative expenses	1		0		3	(29)
Impairment of goodwill	(700)				(1,400)
Impairment of long-lived assets	0		0		0	0
Other income	0		0		0	0
Other deductions	0		0		0	0
Interest income	9		0		16	80
Interest expense and related charges	(361)		(331)		(1,046)	(1,705)
Reorganization items	(40)		(45)		(123)	(468)
Loss before income taxes	(1,093)		(376)		(2,555)	(2,212)
Income tax (expense) benefit	112		114		338	729
Equity earnings of subsidiaries	(536)		225		(850)	321
Net loss	(1,517)		(37)		(3,067)	(1,162)
Other comprehensive income	0		0		1	1
Comprehensive income (loss)	(1,517)		(37)		(3,066)	(1,161)
Other Guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues	1,737		1,807		4,265	4,731
Fuel, purchased power costs and delivery fees	(831)		(868)		(2,090)	(2,256)
Net gain from commodity hedging and trading activities	103		75		226	(28)
Operating costs	(189)		(204)		(598)	(660)
Depreciation and amortization	(198)		(327)		(629)	(983)
Selling, general and administrative expenses	(175)		(163)		(498)	(496)
Impairment of goodwill	0				0
Impairment of long-lived assets	(1,295)		(9)		(1,971)	(30)
Other income	4		5		9	9
Other deductions	(25)		(5)		(86)	(8)
Interest income	35		7		80	270
Interest expense and related charges	(7)		1		(13)	(803)
Reorganization items	1		0		(29)	0
Loss before income taxes	(840)		319		(1,334)	(254)
Income tax (expense) benefit	299		(99)		465	84
Equity earnings of subsidiaries	0		0		4	0
Net loss	(541)		220		(865)	(170)
Other comprehensive income	0		0		0	0
Comprehensive income (loss)	(541)		220		(865)	(170)
Non-guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues	0		0		0	0
Fuel, purchased power costs and delivery fees	0		0		0	0
Net gain from commodity hedging and trading activities	0		0		0	0
Operating costs	0		0		0	0
Depreciation and amortization	0		0		0	0
Selling, general and administrative expenses	0		0		0	(2)
Impairment of goodwill	0				0
Impairment of long-lived assets	0		0		0	0
Other income	0		0		6	2
Other deductions	0		0		0	0
Interest income	0		0		0	0
Interest expense and related charges	0		0		0	0
Reorganization items	0		0		0	0
Loss before income taxes	0		0		6	0
Income tax (expense) benefit	0		0		(2)	0
Equity earnings of subsidiaries	0		0		0	0
Net loss	0		0		4	0
Other comprehensive income	0		0		0	0
Comprehensive income (loss)	0		0		4	0
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Operating revenues	0		0		0	0
Fuel, purchased power costs and delivery fees	0		0		0	0
Net gain from commodity hedging and trading activities	0		0		0	0
Operating costs	0		0		0	0
Depreciation and amortization	0		0		0	0
Selling, general and administrative expenses	0		0		0	0
Impairment of goodwill	0				0
Impairment of long-lived assets	0		0		0	0
Other income	0		0		0	0
Other deductions	0		0		0	0
Interest income	(44)		(7)		(96)	(351)
Interest expense and related charges	43		7		96	1,083
Reorganization items	0		0		0	0
Loss before income taxes	(1)		0		0	732
Income tax (expense) benefit	6		5		15	(241)
Equity earnings of subsidiaries	2,053		(188)		3,913	841
Net loss	2,058		(183)		3,928	1,332
Other comprehensive income	0		0		(1)	(1)
Comprehensive income (loss)	$ 2,058		$ (183)		$ 3,927	$ 1,331