Risk Management and Hedging Activities - Balance Sheet Location (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	$ (80)		$ (77)
Cash collateral, net receivable, offset against derivative positions	85		74
Derivative assets (liabilities), at fair value, net	5		(3)
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	37		48
Cash collateral, net receivable, offset against derivative positions	0		0
Derivative assets (liabilities), at fair value, net	37		48
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	99		94
Cash collateral, net receivable, offset against derivative positions	0		0
Derivative assets (liabilities), at fair value, net	99		94
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(66)		(60)
Cash collateral, net receivable, offset against derivative positions	21		13
Derivative assets (liabilities), at fair value, net	(45)		(47)
Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(150)		(159)
Cash collateral, net receivable, offset against derivative positions	64		61
Derivative assets (liabilities), at fair value, net	(86)		(98)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(65)		(66)
Not Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	37		47
Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	93		86
Not Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(56)		(45)
Not Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(139)		(154)
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(15)		(11)
Designated as Hedging Instrument [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	0		1
Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	6		8
Designated as Hedging Instrument [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(10)		(15)
Designated as Hedging Instrument [Member] | Other Noncurrent Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative, fair value, net	(11)		(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	119		135
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(196)		(206)
Derivative, fair value, net	(77)		(71)
Net Regulatory Asset (Liability), Unrealized Loss (Gain) On Derivative Contracts	162	$ 162	148	$ 195	$ 185	$ 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	16		42
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(1)		(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	93		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	7		5
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(60)		(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	3		2
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(135)		(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	8		6
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(28)		(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	0		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	2		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	6		3
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(17)		(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	22		15
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(10)		(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	22		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	(3)		(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	(7)		(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	6		8
Derivative liability, fair value, gross liability including not subject to master netting arrangement	(1)		(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	6		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	(1)		(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