Assets Held for Sale or Sold (Details) - Beluga River Unit - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2016	Mar. 31, 2016
Long Lived Assets Held For Sale [LineItems]
Net carrying value		$ 78
Amount of PP&E in carrying value of assets		100
Asset retirement obligation of held for sale asset		$ 19
Proceeds from asset dispositions	$ 134

Amount of assets held-for-sale that are not part of a disposal group.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 15
-Paragraph 4
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=6390208&loc=d3e400-110220

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- Definition

Amount classified as property, plant and equipment attributable to disposal group held for sale or disposed of.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=51717284&loc=SL51721673-107760

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=51716988&loc=d3e1107-107759

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-Publisher FASB
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-Section 50
-Paragraph 3
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-URI http://asc.fasb.org/extlink&oid=51719941&loc=d3e2941-110230

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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- Definition

Amount classified as liabilities attributable to disposal group held for sale or disposed of, expected to be disposed of beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=51824106&loc=d3e2510-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=51717284&loc=SL51721673-107760

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=51719941&loc=d3e2941-110230

Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=51716988&loc=SL51721533-107759

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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

No definition available.

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- Definition

Aggregate cash proceeds received from a combination of transactions in which noncurrent assets are sold, which may include the sale of a business, an investment in an affiliate (including an equity method investee), property, plant and equipment and intangible assets. Excludes sales of trading, available-for-sale, and held-to-maturity securities.

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