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v2.4.1.9

Common Equity (Schedule of Capital Contributions) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
IPL [Member] |  |  |
Capital contributions from parent | $ 90.0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_IplMember | $ 120.0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_IplMember | $ 110.0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |  |
Capital contributions from parent | 0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_WplMember | 0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_WplMember | 90.0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_WplMember
Corporate Services [Member] |  |  |
Capital contributions from parent | $ 0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_CorporateServicesMember | $ 0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_CorporateServicesMember | $ 30.0 us-gaap_ProceedsFromContributionsFromParent / dei_LegalEntityAxis = lnt_CorporateServicesMember

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- Definition The cash inflow from parent as a source of financing that is recorded as additional paid in capital. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=lnt_CorporateServicesMember
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- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
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- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
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Data Type: | na
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