Risk Management and Hedging Activities - Balance Sheet Location (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (264)	$ (237)
Cash collateral, net receivable, offset against derivative positions	113	103
Derivative assets (liabilities), at fair value, net	(151)	(134)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	30	28
Cash collateral, net receivable, offset against derivative positions	0	0
Derivative assets (liabilities), at fair value, net	30	28
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	85	75
Cash collateral, net receivable, offset against derivative positions	0	0
Derivative assets (liabilities), at fair value, net	85	75
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(162)	(145)
Cash collateral, net receivable, offset against derivative positions	47	40
Derivative assets (liabilities), at fair value, net	(115)	(105)
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(217)	(195)
Cash collateral, net receivable, offset against derivative positions	66	63
Derivative assets (liabilities), at fair value, net	(151)	(132)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(180)	(188)
Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	30	28
Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	85	72
Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(111)	(109)
Not Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(184)	(179)
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(84)	(49)
Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	0	0
Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	0	3
Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(51)	(36)
Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(33)	(16)
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	118	106
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(292)	(292)
Derivative, fair value, net	(174)	(186)
Net Regulatory Asset (Liability), Unrealized Loss (Gain) On Derivative Contracts	253	250	$ 255	$ 223
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	20	25
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(1)	(4)
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	85	72
Derivative liability, fair value, gross liability including not subject to master netting arrangement	0	0
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	11	7
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(117)	(113)
Commodity derivative [Member] | Not Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	2	2
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(174)	(175)
Commodity derivative [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	3
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(73)	(50)
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	0
Derivative liability, fair value, gross liability including not subject to master netting arrangement	0	0
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	0
Derivative liability, fair value, gross liability including not subject to master netting arrangement	0	0
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	1
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(47)	(33)
Commodity derivative [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	2
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(26)	(17)
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	11	7
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(17)	(9)
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	11	7
Derivative liability, fair value, gross liability including not subject to master netting arrangement	0	0
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	0
Derivative liability, fair value, gross liability including not subject to master netting arrangement	0	0
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	0
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(5)	(3)
Interest Rate Contract [Member] | Not Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	0
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(12)	(6)
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	3
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(11)	(5)
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	0
Derivative liability, fair value, gross liability including not subject to master netting arrangement	0	0
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	3
Derivative liability, fair value, gross liability including not subject to master netting arrangement	0	0
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	0
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(4)	(4)
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative asset, fair value, gross asset including not subject to master netting arrangement	0	0
Derivative liability, fair value, gross liability including not subject to master netting arrangement	$ (7)	$ (1)