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Receivables (Schedule of Allowance for Doubtful Accounts) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | $ 5.1 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent | $ 4.8 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent
IPL [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | 0.4 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_IplMember | 0.7 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | 4.2 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_WplMember | 1.7 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_WplMember
Customer [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | 3.7 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / us-gaap_ValuationAllowancesAndReservesTypeAxis = lnt_CustomerMember | 1.4 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / us-gaap_ValuationAllowancesAndReservesTypeAxis = lnt_CustomerMember
Customer [Member] | IPL [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | 0 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_IplMember / us-gaap_ValuationAllowancesAndReservesTypeAxis = lnt_CustomerMember | 0 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_IplMember / us-gaap_ValuationAllowancesAndReservesTypeAxis = lnt_CustomerMember
Customer [Member] | WPL [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | 3.7 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ValuationAllowancesAndReservesTypeAxis = lnt_CustomerMember | 1.4 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ValuationAllowancesAndReservesTypeAxis = lnt_CustomerMember
Other [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | 1.4 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / us-gaap_ValuationAllowancesAndReservesTypeAxis = us-gaap_OtherCreditDerivativesMember | 3.4 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / us-gaap_ValuationAllowancesAndReservesTypeAxis = us-gaap_OtherCreditDerivativesMember
Other [Member] | IPL [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | 0.4 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_IplMember / us-gaap_ValuationAllowancesAndReservesTypeAxis = us-gaap_OtherCreditDerivativesMember | 0.7 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_IplMember / us-gaap_ValuationAllowancesAndReservesTypeAxis = us-gaap_OtherCreditDerivativesMember
Other [Member] | WPL [Member] |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |
Allowance for doubtful accounts | $ 0.5 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ValuationAllowancesAndReservesTypeAxis = us-gaap_OtherCreditDerivativesMember | $ 0.3 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ValuationAllowancesAndReservesTypeAxis = us-gaap_OtherCreditDerivativesMember

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- Definition A valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 4 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5074-111524 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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