Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012
Variable Interest Entities (Textuals)
Power capacity under capital lease (MW)	236
Capital Lease Agreement With Vie Remaining Term In Years	9
Total payments over remaining terms of the agreement	$ 236.1
Total capacity and lease payments	$ 25.2	$ 29.6

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

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