Variable Interest Entities and Unconsolidated Investments in Power Plants (VIE Texuals) (Details) $ in Millions	3 Months Ended
Mar. 31, 2016 USD ($) MW	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) MW
Variable Interest Entity [Line Items]
Variable interest entity, financial or other support, amount	$ 0	$ 0
Equity method investment, summarized financial information, debt	284		$ 269
Prorata share of equity method investment, summarized financial information, debt	$ 142		$ 135
Greenfield [Member]
Variable Interest Entity [Line Items]
Power generation capacity \| MW	1,038
Equity method investment, ownership percentage	50.00%
Distribution from equity method investee	$ 0	0
Whitby [Member]
Variable Interest Entity [Line Items]
Power generation capacity \| MW	50
Equity method investment, ownership percentage	50.00%
Distribution from equity method investee	$ 0	$ 0
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Power generation capacity \| MW	10,266		10,266

Distribution from equity method investee

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- Definition

Equity Method Investment, Summarized Financial Information, Debt

+ References

No definition available.

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- Definition

Power generation capacity

+ References

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- Definition

Prorata Share of Equity Method Investment, Summarized Financial Information, Debt

+ References

No definition available.

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

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- Definition

The amount of financial or other support the reporting entity has provided (explicitly or implicitly) to the Variable Interest Entity (VIE) that it was not previously contractually required to provide or that the reporting entity intends to provide, including, for example, situations in which the reporting entity assisted the VIE in obtaining another type of support.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=64801991&loc=SL6759159-111685

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- Definition

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.

+ References

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