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v2.4.1.9

Redeemable Preferred Stock (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Temporary Equity [Line Items] |  |  |
Reduction in commercial paper | $ (138.1)us-gaap_ProceedsFromRepaymentsOfShortTermDebt | $ 11.9 us-gaap_ProceedsFromRepaymentsOfShortTermDebt | $ 164.7 us-gaap_ProceedsFromRepaymentsOfShortTermDebt
Preferred stock issuance costs |  | (5.4)us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts |
IPL [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Cumulative preferred stock rate | 5.10%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = lnt_IplMember |  |
Reduction in commercial paper | 0 us-gaap_ProceedsFromRepaymentsOfShortTermDebt / dei_LegalEntityAxis = lnt_IplMember | (76.3)us-gaap_ProceedsFromRepaymentsOfShortTermDebt / dei_LegalEntityAxis = lnt_IplMember | 69.2 us-gaap_ProceedsFromRepaymentsOfShortTermDebt / dei_LegalEntityAxis = lnt_IplMember
Preferred stock issuance costs |  | (5.4)us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts / dei_LegalEntityAxis = lnt_IplMember |
Number of board members to be elected (in members) | 2 lnt_NumberOfBoardMembersToBeElected / dei_LegalEntityAxis = lnt_IplMember |  |
Number of quarterly dividend requirements (in quarters) | 6 lnt_NumberOfQuarterlyDividendRequirements / dei_LegalEntityAxis = lnt_IplMember |  |
IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock issued during period (in shares) |  | 8,000,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / dei_LegalEntityAxis = lnt_IplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FivePointOnePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Cumulative preferred stock rate | 5.10%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = lnt_IplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FivePointOnePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember | 5.10%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = lnt_IplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FivePointOnePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Preferred stock issued during period, value |  | 200 us-gaap_StockIssuedDuringPeriodValueNewIssues / dei_LegalEntityAxis = lnt_IplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FivePointOnePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Preferred stock redemption price per share (in dollars per share) | $ 25 us-gaap_PreferredStockRedemptionPricePerShare / dei_LegalEntityAxis = lnt_IplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FivePointOnePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |  |
IPL [Member] | Redeemable Preferred Stock [Member] | 8.375% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Cumulative preferred stock rate |  | 8.375%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = lnt_IplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_EightPointThreeSevenFivePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Preferred stock redeemed during period (in shares) |  | 6,000,000 us-gaap_StockRedeemedOrCalledDuringPeriodShares / dei_LegalEntityAxis = lnt_IplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_EightPointThreeSevenFivePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Preferred stock redeemed during period, value |  | 150 us-gaap_StockRedeemedOrCalledDuringPeriodValue / dei_LegalEntityAxis = lnt_IplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_EightPointThreeSevenFivePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Alliant Energy and IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock issuance costs |  | 5.0 us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FivePointOnePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Alliant Energy and IPL [Member] | Redeemable Preferred Stock [Member] | 8.375% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock redemption charge |  | 5 us-gaap_PreferredStockRedemptionPremium / dei_LegalEntityAxis = lnt_AlliantEnergyAndIplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_EightPointThreeSevenFivePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
WPL [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Reduction in commercial paper | (183.7)us-gaap_ProceedsFromRepaymentsOfShortTermDebt / dei_LegalEntityAxis = lnt_WplMember | 97.1 us-gaap_ProceedsFromRepaymentsOfShortTermDebt / dei_LegalEntityAxis = lnt_WplMember | 60.9 us-gaap_ProceedsFromRepaymentsOfShortTermDebt / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% through 6.50% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock redeemed during period (in shares) |  | 1,049,225 us-gaap_StockRedeemedOrCalledDuringPeriodShares / dei_LegalEntityAxis = lnt_WplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FourPointFourZeroPercentThroughSixPointFiveZeroPercentMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Preferred stock redeemed during period, value |  | 61 us-gaap_StockRedeemedOrCalledDuringPeriodValue / dei_LegalEntityAxis = lnt_WplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FourPointFourZeroPercentThroughSixPointFiveZeroPercentMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Cumulative preferred stock rate |  | 4.40%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = lnt_WplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FourPointFourZeroPercentMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
WPL [Member] | Redeemable Preferred Stock [Member] | 6.50% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Cumulative preferred stock rate |  | 6.50%us-gaap_PreferredStockDividendRatePercentage / dei_LegalEntityAxis = lnt_WplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_SixPointFiveZeroPercentMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Alliant Energy and WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% through 6.50% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock redemption charge |  | 1 us-gaap_PreferredStockRedemptionPremium / dei_LegalEntityAxis = lnt_AlliantEnergyAndWplMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FourPointFourZeroPercentThroughSixPointFiveZeroPercentMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |
Long-term debt [Member] | IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Reduction in commercial paper |  | $ 40.0 us-gaap_ProceedsFromRepaymentsOfShortTermDebt / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = us-gaap_LongTermDebtMember / lnt_RedeemablePreferredStockByDividendRateAxis = lnt_FivePointOnePercentageMember / us-gaap_StatementClassOfStockAxis = us-gaap_RedeemablePreferredStockMember |

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- Definition Amount of decrease in additional paid in capital (APIC) resulting from direct costs associated with issuing stock. Includes, but is not limited to, legal and accounting fees and direct costs associated with stock issues under a shelf registration. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition The excess of (1) fair value of the consideration transferred to the holders of the preferred stock over (2) the carrying amount of the preferred stock in the registrant's balance sheet, during the accounting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section S99 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27010884&loc=d3e42851-122695 + Details
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- Definition The price per share at which the preferred stock of an entity that has priority over common stock in the distribution of dividends and in the event of liquidation of the entity is redeemed or may be called at. The redemption features of this preferred stock are solely within the control of the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644 + Details
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- Definition The net cash inflow or outflow for borrowing having initial term of repayment within one year or the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 9 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3098-108585 + Details
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