Nature of Operations and Basis of Consolidation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012 customers watts	Dec. 31, 2011	Dec. 31, 2010
Number of megawatts of qualifying facility	35
Number of customers	673,200
Estimated aggregate gross contractual payments through 2024	$ 308.4
Variable interest entity, measure of activity, purchases	$ 21.0	$ 18.4	$ 21.5

The amount of purchases between the reporting entity and the Variable Interest Entity (VIE).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph d

-URI http://asc.fasb.org/extlink&oid=7880789&loc=d3e5747-111685

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS140-4/FIN46(R)-8

-Paragraph C8

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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_VariableInterestEntityActivityBetweenVIEAndEntityPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration