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v2.4.1.9

Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Commercial paper: |  |
Amount outstanding | $ 141.3 us-gaap_CommercialPaper | $ 279.4 us-gaap_CommercialPaper
Weighted average interest rates | 0.40%us-gaap_ShortTermDebtWeightedAverageInterestRate | 0.20%us-gaap_ShortTermDebtWeightedAverageInterestRate
Weighted average remaining maturity | 4 days | 4 days
Available credit facility capacity | 858.7 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity | 720.6 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
IPL [Member] |  |
Commercial paper: |  |
Amount outstanding | 0 us-gaap_CommercialPaper / dei_LegalEntityAxis = lnt_IplMember | 0 us-gaap_CommercialPaper / dei_LegalEntityAxis = lnt_IplMember
Available credit facility capacity | 300.0 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / dei_LegalEntityAxis = lnt_IplMember | 300.0 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Commercial paper: |  |
Amount outstanding | 0 us-gaap_CommercialPaper / dei_LegalEntityAxis = lnt_WplMember | 183.7 us-gaap_CommercialPaper / dei_LegalEntityAxis = lnt_WplMember
Weighted average interest rates |  | 0.10%us-gaap_ShortTermDebtWeightedAverageInterestRate / dei_LegalEntityAxis = lnt_WplMember
Weighted average remaining maturity |  | 6 days
Available credit facility capacity | $ 400.0 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / dei_LegalEntityAxis = lnt_WplMember | $ 216.3 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / dei_LegalEntityAxis = lnt_WplMember

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- Definition Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(2)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph a(3) -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 2 -Article 9 + Details
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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition Weighted average interest rate of short-term debt outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph b -Article 5 + Details
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