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Property, Plant and Equipment (Equity and Debt AFUDC) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 34.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions | $ 30.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions | $ 21.9 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions
Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 11.7 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember | 10.5 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember | 7.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember
Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 23.1 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember | 20.3 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember | 14.1 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember
IPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 25.9 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember | 21.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember | 8.4 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 8.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember / dei_LegalEntityAxis = lnt_IplMember | 7.2 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember / dei_LegalEntityAxis = lnt_IplMember | 3.2 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 17.1 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember | 13.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember | 5.2 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_IplMember / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember
WPL [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 8.9 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember | 9.8 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember | 13.5 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Debt [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | 2.9 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember / dei_LegalEntityAxis = lnt_WplMember | 3.3 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember / dei_LegalEntityAxis = lnt_WplMember | 4.6 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / us-gaap_DerivativeByNatureAxis = us-gaap_DebtMember / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Equity [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Allowance for funds used during construction | $ 6.0 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember | $ 6.5 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember | $ 8.9 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = lnt_WplMember / us-gaap_TradingActivityByTypeAxis = us-gaap_EquityMember

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- Definition Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500269&loc=d3e45485-110386 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44278-110382 + Details
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