Derivative Instruments - Derivative Instruments on the Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Designated as cash flow hedges
Total designated as cash flow hedges, net of tax	$ (2)	$ (4)	$ (1)	$ (4)
Not designated as hedges
Income tax	33	(1)	26	9
Total not designated as hedges, net of tax	(55)	3	(43)	(13)
Total losses on derivative instruments, net of tax	(57)	(1)	(44)	(17)
Cash Flow Hedging [Member]
Designated as cash flow hedges
Total designated as cash flow hedges, net of tax	(2)	(4)	(1)	(4)
Natural Gas Contracts [Member] \| Operating Revenues [Member]
Not designated as hedges
Natural gas contracts - net value adjustments	(93)	3	(73)	(21)
Natural Gas Contracts [Member] \| Cost of Sales [Member]
Not designated as hedges
Natural gas contracts - net value adjustments	5	1	4	(1)
Natural Gas Contracts [Member] \| Cost of Sales [Member] \| Cash Flow Hedging [Member]
Designated as cash flow hedges
Net gain (loss) reclassified from OCI	(1)	(3)	(1)	(4)
Natural Gas Contracts [Member] \| Operation and Maintenance Expense [Member] \| Cash Flow Hedging [Member]
Designated as cash flow hedges
Net gain (loss) reclassified from OCI	0	(1)	0	(1)
Interest Rate Swaps [Member] \| Interest Expense [Member] \| Cash Flow Hedging [Member]
Designated as cash flow hedges
Net gain (loss) reclassified from OCI	$ (1)	$ 0	$ 0	$ 1

Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net of Tax

No definition available.

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- Definition

Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Tax

+ References

No definition available.

+ Details

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- References

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- References

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- Definition

Total of gain (loss) in Income statement related to derivative instruments.

+ References

No definition available.

+ Details

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- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624177-113959

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- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

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- Definition

Amount after tax of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e689-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph c
-URI http://asc.fasb.org/extlink&oid=56946850&loc=d3e41638-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580

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