Risk Management and Hedging Activities - Balance Sheet Location (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (117)	$ (77)
Cash collateral, net receivable, offset against derivative positions	88	74
Derivative assets (liabilities), at fair value, net	(29)	(3)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	29	48
Cash collateral, net receivable, offset against derivative positions	0	0
Derivative assets (liabilities), at fair value, net	29	48
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	98	94
Cash collateral, net receivable, offset against derivative positions	0	0
Derivative assets (liabilities), at fair value, net	98	94
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(66)	(60)
Cash collateral, net receivable, offset against derivative positions	16	13
Derivative assets (liabilities), at fair value, net	(50)	(47)
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(178)	(159)
Cash collateral, net receivable, offset against derivative positions	72	61
Derivative assets (liabilities), at fair value, net	(106)	(98)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(98)	(66)
Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	28	47
Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	90	86
Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(54)	(45)
Not Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(162)	(154)
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(19)	(11)
Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	1	1
Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	8	8
Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(12)	(15)
Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(16)	(5)
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	116	135
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(216)	(206)
Derivative, fair value, net	(100)	(71)
Net Regulatory Asset (Liability), Unrealized Loss (Gain) On Derivative Contracts	180	148	$ 253	$ 250
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	19	42
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(4)	(10)
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	90	86
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	6	5
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(55)	(46)
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	1	2
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(157)	(150)
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	6	6
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(29)	(22)
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	1	1
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	3	2
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(11)	(14)
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	2	3
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(18)	(8)
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	13	15
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(11)	(10)
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	13	15
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)	(4)
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	(6)	(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	8	8
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(4)	(3)
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	8	8
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)	(3)
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	$ 0	$ 0