Derivative Instruments - Derivative Instruments on the Condensed Consolidated Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Assets	$ 942	$ 1,212
Liabilities	(866)	(1,151)
Gross amounts offset on our Consolidated Balance Sheets	(724)	(925)
Gross amounts offset on our Consolidated Balance Sheets	(820)	(1,058)
Net amounts of assets and liabilities presented on our Consolidated Balance Sheets	218	287
Net amounts of assets and liabilities presented on our Consolidated Balance Sheets	(46)	(93)
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	12	6
Liabilities	(7)	(12)
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	930	1,206
Liabilities	(859)	(1,139)
Current Natural Gas Contracts [Member] \| Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	3	6
Liabilities	(5)	(11)
Current Natural Gas and Weather Contracts [Member] \| Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	751	1,061
Liabilities	(672)	(1,020)
Long-Term Natural Gas Contracts [Member] \| Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	0	0
Liabilities	(2)	(1)
Long-Term Natural Gas Contracts [Member] \| Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	179	145
Liabilities	(187)	(119)
Current Interest Rate Swap Agreements [Member] \| Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	9	0
Liabilities	$ 0	$ 0

Fair value, before 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. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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-Name Accounting Standards Codification
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Fair value, after offset of derivative liability, of 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, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

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-Publisher FASB
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Fair value, after offset of derivative asset, of 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, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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