Asset Impairments (Narrative) (Detail) (USD $)	2 Months Ended	3 Months Ended	6 Months Ended
Feb. 29, 2012	Mar. 31, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Percentage decrease in market price	14.00%
Noncash impairment charge		$ 628,000,000	$ 2,000,000	$ 628,000,000	$ 2,000,000
Merchant Generation [Member]
Long-Lived asset carrying value in excess of estimated fair value		$ 1,000,000,000

The charge against earnings resulting from the aggregate write down of all assets from their carrying value to their fair value.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 45, 46, 47

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 45

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228

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