Risk Management Effect of Derivative Contracts on the Income Statement (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Interest Rate Swap [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) recognized in income on derivative	$ (211)	$ 194	$ (294)	$ 81
Interest Rate Swap [Member] | Interest expense [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) recognized in income on derivative	(211)	194	(294)	81
Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain to be reclassified within twelve months			24
Commodity Contract [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	3	11	10	(20)
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	9	0	6	(3)
Commodity Contract [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) recognized in income on derivative	211	(194)	294	(81)
Commodity Contract [Member] | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) recognized in OCI on derivative (effective portion)	70	303	29	189
Commodity Contract [Member] | Interest expense [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) recognized in income on derivative	211	(194)	294	(81)
Commodity Contract [Member] | Revenues-Natural Gas Sales [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	0	2	0	2
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	0	0	0	0
Commodity Contract [Member] | Revenues-Product sales and other [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	8	(2)	15	(31)
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	9	0	6	(3)
Commodity Contract [Member] | Gas purchases and other costs of sales [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss) reclassified from Accumulated OCI into income (effective portion)	(5)	11	(5)	9
Amount of gain/(loss) recognized in income on derivative (ineffective portion and amount excluded from effectiveness testing)	$ 0	$ 0	$ 0	$ 0