Fair Value of Intangible Assets (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended		12 Months Ended
Jun. 30, 2012	Dec. 31, 2011 power_purchase_contract	Dec. 31, 2010 Power Purchase Contracts [Member]	Dec. 31, 2011 WNP-3 BPA Exchange Power Contract [Member]	Dec. 31, 2011 WNP-3 BPA Exchange Power Contract [Member] Maximum	Dec. 31, 2011 Rock Island Power Contract [Member]	Dec. 31, 2010 SO2 Emission Allowance Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Number of purchased power contracts impaired		2
Intangible asset carrying value			$ 864.7	$ 1.9		$ 44.9	$ 7.9
Intangible asset fair value					0	9.8
Impairment of intangible assets	$ 16.6		$ 105.8	$ 1.9		$ 35.1	$ 7.9

The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 3

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7658586&loc=d3e16373-109275

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 142

-Paragraph 46

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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