Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012 Megawatt	Dec. 31, 2011	Dec. 31, 2010
Variable Interest Entities (Textuals)
Power capacity from a gas-fired cogeneration facility under capital lease (MW)	236
Capital lease agreement with VIE remaining term in years	10
Total payments over remaining terms of the two agreements	$ 256.3
Total capacity and minimum lease payments	$ 45.8	$ 65.9	$ 64.2

Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service. If the utility is obligated to pay certain minimum amounts which cover debt service requirements whether or not the plant is operating, the amount can be disclosed here.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 10.D)

-URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030

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

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 10

-Section D

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration