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Income Taxes (Schedule Of Uncertain Tax Positions) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Uncertain Tax Positions [Roll Forward] |  |  |  |  |  |
Balance, January 1 | $ 0 us-gaap_UnrecognizedTaxBenefits |  | $ 0.7 us-gaap_UnrecognizedTaxBenefits |  | $ 23.5 us-gaap_UnrecognizedTaxBenefits |
Additions based on tax positions related to the current year | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions |  | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions |  | 0.7 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions |
Reductions for tax positions of prior years | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions | [1] | (0.7)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions | [1] | (23.5)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions | [1]
Balance, December 31 | 0 us-gaap_UnrecognizedTaxBenefits |  | 0 us-gaap_UnrecognizedTaxBenefits |  | 0.7 us-gaap_UnrecognizedTaxBenefits |
Statute of limitations, expiration period from extended due date of federal tax return | 3 years |  |  |  |  |
Statute of limitations, expiration period from extended due date of Iowa tax return | 3 years |  |  |  |  |
Statute of limitations, expiration period from extended due date of Wisconsin tax return | 4 years |  |  |  |  |
IPL [Member] |  |  |  |  |  |
Uncertain Tax Positions [Roll Forward] |  |  |  |  |  |
Balance, January 1 | 0 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_IplMember |  | 10.9 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_IplMember |
Additions based on tax positions related to the current year | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / dei_LegalEntityAxis = lnt_IplMember |
Reductions for tax positions of prior years | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions / dei_LegalEntityAxis = lnt_IplMember | [1] | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions / dei_LegalEntityAxis = lnt_IplMember | [1] | (10.9)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions / dei_LegalEntityAxis = lnt_IplMember | [1]
Balance, December 31 | 0 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_IplMember |
WPL [Member] |  |  |  |  |  |
Uncertain Tax Positions [Roll Forward] |  |  |  |  |  |
Balance, January 1 | 0 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_WplMember |  | 0.7 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_WplMember |  | 12.6 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_WplMember |
Additions based on tax positions related to the current year | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / dei_LegalEntityAxis = lnt_WplMember |  | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / dei_LegalEntityAxis = lnt_WplMember |  | 0.7 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions / dei_LegalEntityAxis = lnt_WplMember |
Reductions for tax positions of prior years | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions / dei_LegalEntityAxis = lnt_WplMember | [1] | (0.7)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions / dei_LegalEntityAxis = lnt_WplMember | [1] | (12.6)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions / dei_LegalEntityAxis = lnt_WplMember | [1]
Balance, December 31 | $ 0 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_WplMember |  | $ 0 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_WplMember |  | $ 0.7 us-gaap_UnrecognizedTaxBenefits / dei_LegalEntityAxis = lnt_WplMember |
[1] | In 2012, the reductions for tax positions of prior years were due to the finalization of Alliant Energy’s federal income tax return audits for calendar years 2005 through 2009.

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- Definition Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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