Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilites) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Offsetting Assets [Line Items]
Total fair value of derivatives	$ 1,178	$ 833
Derivative Asset, Fair Value, Gross Liability	(1,006)	(707)
Net Amounts of Assets Presented in the Statements of Financial Position	172us-gaap_DerivativeAssets	126us-gaap_DerivativeAssets
Fair Value Gross Amounts of Recognized (Liabilities)	(1,110)	(936)
Derivative Liability, Fair Value, Gross Asset	1,025	698
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	(85)us-gaap_DerivativeLiabilities	(238)us-gaap_DerivativeLiabilities
Natural Gas Commodity Contract [Member]
Offsetting Assets [Line Items]
Total fair value of derivatives	787	396
Derivative Asset, Fair Value, Gross Liability	(681)	(382)
Net Amounts of Assets Presented in the Statements of Financial Position	106	14
Fair Value Gross Amounts of Recognized (Liabilities)	(718)	(503)
Derivative Liability, Fair Value, Gross Asset	679	395
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	(39)	(108)
Electricity Commodity Contract [Member]
Offsetting Assets [Line Items]
Total fair value of derivatives	342	400
Derivative Asset, Fair Value, Gross Liability	(280)	(291)
Net Amounts of Assets Presented in the Statements of Financial Position	62	109
Fair Value Gross Amounts of Recognized (Liabilities)	(342)	(398)
Derivative Liability, Fair Value, Gross Asset	298	269
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	(44)	(129)
Other Commodity Contract [Member]
Offsetting Assets [Line Items]
Total fair value of derivatives	45	37
Derivative Asset, Fair Value, Gross Liability	(42)	(34)
Net Amounts of Assets Presented in the Statements of Financial Position	3	3
Fair Value Gross Amounts of Recognized (Liabilities)	(45)	(34)
Derivative Liability, Fair Value, Gross Asset	45	34
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	0	0
Other Derivative Contract [Member]
Offsetting Assets [Line Items]
Total fair value of derivatives	4	0
Derivative Asset, Fair Value, Gross Liability	(3)	0
Net Amounts of Assets Presented in the Statements of Financial Position	1	0
Fair Value Gross Amounts of Recognized (Liabilities)	(5)	(1)
Derivative Liability, Fair Value, Gross Asset	3	0
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	$ (2)	$ (1)

Fair value of liability associated with 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, and obligation to return cash collateral under master netting arrangements.

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