Financial and Other Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	$ 843	$ 1,016
Derivative Liability Fair Value Gross Liability	(815)	(1,020)
Derivative Assets, Current	72	108
Derivative Assets, Noncurrent	26	39
Derivative Liabilities, Current	(58)	(125)
Derivative Liabilities, Noncurrent	(21)	(26)
Estimate of Fair Value, Fair Value Disclosure [Member]
Derivatives designated as hedging instruments:
Derivative Assets, Current	711	862
Derivative Assets, Noncurrent	132	154
Derivative Liabilities, Current	(668)	(879)
Derivative Liabilities, Noncurrent	(147)	(141)
Interest rate contracts [Member]
Derivatives designated as hedging instruments:
Derivative Liability Fair Value Gross Liability	0	(1)
Natural Gas Commodity Contract [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	483	645
Derivative Liability Fair Value Gross Liability	(471)	(661)
Electricity [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	298	360
Derivative Liability Fair Value Gross Liability	(287)	(351)
Designated as Hedging Instrument [Member] \| Interest rate contracts [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	0	0
Derivative Liability Fair Value Gross Liability	0	(1)
Not Designated as Hedging Instrument [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	843	1,016
Derivative Liability Fair Value Gross Liability	(815)	(1,019)
Not Designated as Hedging Instrument [Member] \| Natural Gas Commodity Contract [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	483	645
Derivative Liability Fair Value Gross Liability	(471)	(661)
Not Designated as Hedging Instrument [Member] \| Electricity [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	298	360
Derivative Liability Fair Value Gross Liability	(287)	(351)
Not Designated as Hedging Instrument [Member] \| Other [Member]
Derivatives designated as hedging instruments:
Derivative Asset Fair Value Gross Asset	62	11
Derivative Liability Fair Value Gross Liability	$ (57)	$ (7)

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References+ Details

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References+ Details