Acadia Transactions (Details) (USD $) In Millions, unless otherwise specified	Apr. 29, 2011
APH [Member] \| Acadia Unit 2 and Facilities [Member]
Significant Acquisitions and Disposals [Line Items]
Recognized gain on transaction	$ 62.0
Indemnification against third parties	10.9
Third parties' share of entity's liabilities (in hundredths)	50.00%
Net adjustment to equity income	26.2
Impairment of investment	45.9
Capitalized interest and other	19.7
Acadia Power Partners [Member] \| Acadia Unit 2 and Facilities [Member]
Significant Acquisitions and Disposals [Line Items]
Proceeds from plant disposition	$ 298.8
Acadia Power Partners [Member] \| APH [Member]
Significant Acquisitions and Disposals [Line Items]
Ownership interest received (in hundredths)	100.00%
Cajun [Member] \| APH [Member]
Significant Acquisitions and Disposals [Line Items]
Ownership interest exchanged (in hundredths)	50.00%

Capitalized interest and other in the investee on the equity method of accounting, from inception to date.

No definition available.

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- Definition

Impairment of investment in the investee on the equity method of accounting from inception to date.

+ References

No definition available.

+ Details

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- Definition

The amount of indemnification liability recognized at the acquisition date.

+ References

No definition available.

+ Details

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- Definition

This item represents a net adjustment to the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ References

No definition available.

+ Details

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- Definition

Ownership interest exchanged

+ References

No definition available.

+ Details

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- Definition

The third parties' share of entity's liabilities and other obligations.

+ References

No definition available.

+ Details

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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 Statement of Financial Accounting Standard (FAS)

-Number 159

-Paragraph 18

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 18

-Paragraph 20

-Subparagraph a (1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: 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=6382943&loc=d3e33918-111571

+ Details

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- Definition

The value of all consideration given or received by the Entity in the significant acquisition or disposal.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

The difference between the value of the consideration received by the Entity on the sale or disposal and the consideration received by the Entity, including any tax related impact.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2941-110230

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760

+ Details

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