Acquisitions and Divestitures Acquisition of Bosque (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012 MW	Dec. 31, 2011 MW
Business Acquisition [Line Items]
Power generation capacity	10,500	11,391
Bosque Energy Center [Member]
Business Acquisition [Line Items]
Business Acquisition, Purchase Price Allocation, Assets Acquired	432
Power generation capacity	800
Block one power generation capacity	250
Block two power generation capacity	550

The amount of acquisition cost of a business combination allocated to assets acquired.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141

-Paragraph 37

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

Name:	us-gaap_BusinessAcquisitionPurchasePriceAllocationAssetsAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant