Goodwill and Other Intangibles Goodwill and Other Intangibles - (Intangibles - Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 3,811	$ 3,835		$ 3,969
Finite-Lived Intangible Assets, Purchases	84	137
Finite-lived Intangible Assets Acquired	18
Finite-Lived Intangible Assets, Usage	(45)	95
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	10	(154)
Impairment of Intangible Assets, Finite-lived	64	11
Finite-Lived Intangible Assets, Other Changes	(7)	11
Finite-Lived Intangible Assets, Accumulated Amortization	1,775	1,525	[1]
Finite-Lived Intangible Assets, Net	2,036	2,310
Amortization of Intangible Assets	278	277		268
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	198
Finite-Lived Intangible Assets, Amortization Expense, Year Two	136
Finite-Lived Intangible Assets, Amortization Expense, Year Three	129
Finite-Lived Intangible Assets, Amortization Expense, Year Four	112
Finite-Lived Intangible Assets, Amortization Expense, Year Five	110
Other Assets, Noncurrent, Emission Allowances	39
Emission Allowances [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	929	920		1,018
Finite-Lived Intangible Assets, Purchases	50	77
Finite-lived Intangible Assets Acquired	0
Finite-Lived Intangible Assets, Usage	(1)	(33)
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	10	154
Impairment of Intangible Assets, Finite-lived	23	0
Finite-Lived Intangible Assets, Other Changes	(7)	12
Finite-Lived Intangible Assets, Accumulated Amortization	605	502	[1]
Finite-Lived Intangible Assets, Net	324	418
Amortization of Intangible Assets	113	99		124
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	82
Finite-Lived Intangible Assets, Amortization Expense, Year Two	33
Finite-Lived Intangible Assets, Amortization Expense, Year Three	31
Finite-Lived Intangible Assets, Amortization Expense, Year Four	16
Finite-Lived Intangible Assets, Amortization Expense, Year Five	16
Energy Supply Contracts [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	54	54		54
Finite-Lived Intangible Assets, Purchases	0	0
Finite-lived Intangible Assets Acquired	0
Finite-Lived Intangible Assets, Usage	0	0
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	0	0
Impairment of Intangible Assets, Finite-lived	0	0
Finite-Lived Intangible Assets, Other Changes	0	0
Finite-Lived Intangible Assets, Accumulated Amortization	54	47	[1]
Finite-Lived Intangible Assets, Net	0	7
Amortization of Intangible Assets	7	5		6
Fuel Contracts [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	72	72		72
Finite-Lived Intangible Assets, Purchases	0	0
Finite-lived Intangible Assets Acquired	0
Finite-Lived Intangible Assets, Usage	0	0
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	0	0
Impairment of Intangible Assets, Finite-lived	0	0
Finite-Lived Intangible Assets, Other Changes	0	0
Finite-Lived Intangible Assets, Accumulated Amortization	67	65	[1]
Finite-Lived Intangible Assets, Net	5	7
Amortization of Intangible Assets	2	2		2
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	1
Finite-Lived Intangible Assets, Amortization Expense, Year Two	0
Finite-Lived Intangible Assets, Amortization Expense, Year Three	0
Finite-Lived Intangible Assets, Amortization Expense, Year Four	0
Finite-Lived Intangible Assets, Amortization Expense, Year Five	0
Customer Contracts [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	16	16		16
Finite-Lived Intangible Assets, Purchases	0	0
Finite-lived Intangible Assets Acquired	0
Finite-Lived Intangible Assets, Usage	0	0
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	0	0
Impairment of Intangible Assets, Finite-lived	0	0
Finite-Lived Intangible Assets, Other Changes	0	0
Finite-Lived Intangible Assets, Accumulated Amortization	8	6	[1]
Finite-Lived Intangible Assets, Net	8	10
Amortization of Intangible Assets	2	2		0
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	1
Finite-Lived Intangible Assets, Amortization Expense, Year Two	1
Finite-Lived Intangible Assets, Amortization Expense, Year Three	1
Finite-Lived Intangible Assets, Amortization Expense, Year Four	1
Finite-Lived Intangible Assets, Amortization Expense, Year Five	1
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	816	834		831
Finite-Lived Intangible Assets, Purchases	0	3
Finite-lived Intangible Assets Acquired	0
Finite-Lived Intangible Assets, Usage	0	0
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	0	0
Impairment of Finite-Lived Intangible Assets Accumulated Amortization	10
Impairment of Intangible Assets, Finite-lived	18	0
Impairment of Intangible Assets, Finite-Lived, Net	8
Finite-Lived Intangible Assets, Other Changes	0	0
Finite-Lived Intangible Assets, Accumulated Amortization	663	624	[1]
Finite-Lived Intangible Assets, Net	153	210
Amortization of Intangible Assets	49	67		70
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	26
Finite-Lived Intangible Assets, Amortization Expense, Year Two	14
Finite-Lived Intangible Assets, Amortization Expense, Year Three	10
Finite-Lived Intangible Assets, Amortization Expense, Year Four	8
Finite-Lived Intangible Assets, Amortization Expense, Year Five	6
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Impairment of Finite-Lived Intangible Assets Accumulated Amortization	8
Impairment of Intangible Assets, Finite-Lived, Net	15
Marketing Partnerships [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	88	88		88
Finite-Lived Intangible Assets, Purchases	0	0
Finite-lived Intangible Assets Acquired	0
Finite-Lived Intangible Assets, Usage	0	0
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	0	0
Impairment of Intangible Assets, Finite-lived	0	0
Finite-Lived Intangible Assets, Other Changes	0	0
Finite-Lived Intangible Assets, Accumulated Amortization	49	41	[1]
Finite-Lived Intangible Assets, Net	39	47
Amortization of Intangible Assets	8	14		15
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	5
Finite-Lived Intangible Assets, Amortization Expense, Year Two	5
Finite-Lived Intangible Assets, Amortization Expense, Year Three	4
Finite-Lived Intangible Assets, Amortization Expense, Year Four	4
Finite-Lived Intangible Assets, Amortization Expense, Year Five	4
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	342	342		353
Finite-Lived Intangible Assets, Purchases	0	0
Finite-lived Intangible Assets Acquired	0
Finite-Lived Intangible Assets, Usage	0	0
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	0	0
Impairment of Intangible Assets, Finite-lived	0	6
Finite-Lived Intangible Assets, Other Changes	0	(5)
Finite-Lived Intangible Assets, Accumulated Amortization	159	137	[1]
Finite-Lived Intangible Assets, Net	183	205
Amortization of Intangible Assets	22	23		21
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	23
Finite-Lived Intangible Assets, Amortization Expense, Year Two	23
Finite-Lived Intangible Assets, Amortization Expense, Year Three	23
Finite-Lived Intangible Assets, Amortization Expense, Year Four	23
Finite-Lived Intangible Assets, Amortization Expense, Year Five	23
PPA [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	1,264	1,264		1,270
Finite-Lived Intangible Assets, Purchases	0	0
Finite-lived Intangible Assets Acquired	0
Finite-Lived Intangible Assets, Usage	0	0
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	0	0
Impairment of Intangible Assets, Finite-lived	0	0
Finite-Lived Intangible Assets, Other Changes	0	(6)
Finite-Lived Intangible Assets, Accumulated Amortization	138	75	[1]
Finite-Lived Intangible Assets, Net	1,126	1,189
Amortization of Intangible Assets	63	50		24
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	57
Finite-Lived Intangible Assets, Amortization Expense, Year Two	57
Finite-Lived Intangible Assets, Amortization Expense, Year Three	57
Finite-Lived Intangible Assets, Amortization Expense, Year Four	57
Finite-Lived Intangible Assets, Amortization Expense, Year Five	57
Other Finite Lived Intangible Assets Member
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	230	245		267
Finite-Lived Intangible Assets, Purchases	34	57
Finite-lived Intangible Assets Acquired	18
Finite-Lived Intangible Assets, Usage	(44)	(62)
Finite-Lived Intangible Assets, Write-off of Fully Amortized Intangible Assets	0	0
Impairment of Intangible Assets, Finite-lived	23	5
Finite-Lived Intangible Assets, Other Changes	0	(12)
Finite-Lived Intangible Assets, Accumulated Amortization	32	28	[1]
Finite-Lived Intangible Assets, Net	198	217
Amortization of Intangible Assets	12	$ 15		$ 6
Finite-Lived Intangible Assets, Net - 5 year Amortization [Abstract]
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	3
Finite-Lived Intangible Assets, Amortization Expense, Year Two	3
Finite-Lived Intangible Assets, Amortization Expense, Year Three	3
Finite-Lived Intangible Assets, Amortization Expense, Year Four	3
Finite-Lived Intangible Assets, Amortization Expense, Year Five	$ 3

[1] Adjusted for write-off of fully amortized emission allowances of $154 million.