XML 51 R89.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Debt (Long-term Debt) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | $ 3,802.8 us-gaap_LongTermDebt |  | $ 3,348.4 us-gaap_LongTermDebt |  |
Current maturities | (183.0)us-gaap_LongTermDebtCurrent |  | (358.5)us-gaap_LongTermDebtCurrent |  |
Unamortized debt (discount) and premium, net | (13.1)us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet |  | (12.1)us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet |  |
Long-term debt, net | 3,606.7 us-gaap_LongTermDebtNoncurrent |  | 2,977.8 us-gaap_LongTermDebtNoncurrent |  |
Payments to retire long-term debt | (358.5)us-gaap_RepaymentsOfLongTermDebt |  | (1.5)us-gaap_RepaymentsOfLongTermDebt |  | (1.4)us-gaap_RepaymentsOfLongTermDebt
Senior Debentures [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 1,775.0 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 1,525.0 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 3.3% senior debenture, due 2015 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 150.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointThreePercentSeniorDebenturesDueTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 150.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointThreePercentSeniorDebenturesDueTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 3.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointThreePercentSeniorDebenturesDueTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 3.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointThreePercentSeniorDebenturesDueTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointEightSevenFivePercentSeniorDebenturesDueTwoThousandEighteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointEightSevenFivePercentSeniorDebenturesDueTwoThousandEighteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 5.875%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointEightSevenFivePercentSeniorDebenturesDueTwoThousandEighteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 5.875%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointEightSevenFivePercentSeniorDebenturesDueTwoThousandEighteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SevenPointTwoFivePercentSeniorDebenturesDueTwoThousandEighteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SevenPointTwoFivePercentSeniorDebenturesDueTwoThousandEighteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 7.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SevenPointTwoFivePercentSeniorDebenturesDueTwoThousandEighteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 7.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SevenPointTwoFivePercentSeniorDebenturesDueTwoThousandEighteenMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 200.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointSixFivePercentSeniorDebenturesDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 200.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointSixFivePercentSeniorDebenturesDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 3.65%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointSixFivePercentSeniorDebenturesDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 3.65%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointSixFivePercentSeniorDebenturesDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember | [1] | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember | [1] |
Interest rate | 3.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |  |  |
Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointFivePercentSeniorDebenturesDueTwoThousandTwentyFiveMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointFivePercentSeniorDebenturesDueTwoThousandTwentyFiveMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 5.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointFivePercentSeniorDebenturesDueTwoThousandTwentyFiveMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 5.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointFivePercentSeniorDebenturesDueTwoThousandTwentyFiveMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointFourFivePercentSeniorDebenturesDueTwoThousandThirtyThreeMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointFourFivePercentSeniorDebenturesDueTwoThousandThirtyThreeMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 6.45%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointFourFivePercentSeniorDebenturesDueTwoThousandThirtyThreeMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 6.45%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointFourFivePercentSeniorDebenturesDueTwoThousandThirtyThreeMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 125.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointThreePercentSeniorDebenturesDueTwoThousandThirtyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 125.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointThreePercentSeniorDebenturesDueTwoThousandThirtyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 6.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointThreePercentSeniorDebenturesDueTwoThousandThirtyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 6.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointThreePercentSeniorDebenturesDueTwoThousandThirtyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 300.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentSeniorDebenturesDueTwoThousandThirtyNineMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 300.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentSeniorDebenturesDueTwoThousandThirtyNineMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentSeniorDebenturesDueTwoThousandThirtyNineMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentSeniorDebenturesDueTwoThousandThirtyNineMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointSevenPercentSeniorDebenturesDueTwoThousandFortyThreeMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointSevenPercentSeniorDebenturesDueTwoThousandFortyThreeMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 4.70%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointSevenPercentSeniorDebenturesDueTwoThousandFortyThreeMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 4.70%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointSevenPercentSeniorDebenturesDueTwoThousandFortyThreeMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Debentures [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 1,550.0 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 1,300.0 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Debentures [Member] | 5% debenture, due 2019 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentDebentureDueTwoThousandNineteenMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentDebentureDueTwoThousandNineteenMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentDebentureDueTwoThousandNineteenMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentDebentureDueTwoThousandNineteenMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Debentures [Member] | 4.6% debenture, due 2020 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 150.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointSixPercentDebentureDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 150.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointSixPercentDebentureDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 4.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointSixPercentDebentureDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 4.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointSixPercentDebentureDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Debentures [Member] | 2.25% debenture, due 2022 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TwoPointTwoFivePercentDebentureDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TwoPointTwoFivePercentDebentureDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_TwoPointTwoFivePercentDebentureDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_TwoPointTwoFivePercentDebentureDueTwoThousandTwentyMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Debentures [Member] | 6.25% debenture, due 2034 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentDebentureDueTwoThousandThirtyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentDebentureDueTwoThousandThirtyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentDebentureDueTwoThousandThirtyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentDebentureDueTwoThousandThirtyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Debentures [Member] | 6.375% debenture, due 2037 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 300.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointThreeSevenFivePercentDebentureDueTwoThousandThirtySevenMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 300.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointThreeSevenFivePercentDebentureDueTwoThousandThirtySevenMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 6.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointThreeSevenFivePercentDebentureDueTwoThousandThirtySevenMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 6.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointThreeSevenFivePercentDebentureDueTwoThousandThirtySevenMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Debentures [Member] | 7.6% debenture, due 2038 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SevenPointSixPercentDebentureDueTwoThousandThirtyEightMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SevenPointSixPercentDebentureDueTwoThousandThirtyEightMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 7.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SevenPointSixPercentDebentureDueTwoThousandThirtyEightMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 7.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SevenPointSixPercentDebentureDueTwoThousandThirtyEightMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Debentures [Member] | 4.1% debenture, due 2044 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointOnePercentDebentureDueTwoThousandFortyFourMemberMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember | [2] | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointOnePercentDebentureDueTwoThousandFortyFourMemberMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember | [2] |
Interest rate | 4.10%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointOnePercentDebentureDueTwoThousandFortyFourMemberMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |  |  |
Pollution Control Revenue Bonds [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 30.6 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 77.5 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond, due 2015 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 16.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenAndTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember | [3] | 24.5 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenAndTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember | [3] |
Interest rate | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenAndTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenAndTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond, due 2015 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 14.6 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointThreeSevenFivePercentPollutionControlRevenueBondsDueTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 14.6 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointThreeSevenFivePercentPollutionControlRevenueBondsDueTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
Interest rate | 5.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointThreeSevenFivePercentPollutionControlRevenueBondsDueTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 5.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointThreeSevenFivePercentPollutionControlRevenueBondsDueTwoThousandFifteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember | [4] | 38.4 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember | [4] |
Interest rate |  |  | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
Other Long Term Debt [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 447.2 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_OtherLongTermDebtMember |  | 445.9 us-gaap_LongTermDebt / us-gaap_LongtermDebtTypeAxis = lnt_OtherLongTermDebtMember |  |
Term Loan Credit Agreement [Member] | Term loan credit agreement through 2016, 1% at December 31, 2014 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwentySixteenMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember | [5] | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwentySixteenMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember | [5] |
Interest rate | 1.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwentySixteenMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember |  |  |  |
Senior Notes [Member] | 3.45% senior notes, due 2022 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointFourFivePercentSeniorNotesDueTwoThousandTwentyTwoMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember |  | 75.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointFourFivePercentSeniorNotesDueTwoThousandTwentyTwoMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember |  |
Interest rate | 3.45%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointFourFivePercentSeniorNotesDueTwoThousandTwentyTwoMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember |  | 3.45%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointFourFivePercentSeniorNotesDueTwoThousandTwentyTwoMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember |  |
Senior Notes [Member] | 4% senior notes, due 2014 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPercentSeniorNotesDueTwoThousandFourteenMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember | [5] | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPercentSeniorNotesDueTwoThousandFourteenMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember | [5] |
Interest rate |  |  | 4.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPercentSeniorNotesDueTwoThousandFourteenMember / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember |  |
Senior Secured Notes [Member] | 5.06% senior secured notes, due 2015 to 2024 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 58.9 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointZeroSixPercentOtherSeniorNotesDueTwoThousandTwelveToTwoThousandTwentyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorSecuredNotesMember |  | 60.5 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointZeroSixPercentOtherSeniorNotesDueTwoThousandTwelveToTwoThousandTwentyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorSecuredNotesMember |  |
Interest rate | 5.06%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointZeroSixPercentOtherSeniorNotesDueTwoThousandTwelveToTwoThousandTwentyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorSecuredNotesMember |  | 5.06%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointZeroSixPercentOtherSeniorNotesDueTwoThousandTwelveToTwoThousandTwentyFourMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorSecuredNotesMember |  |
Other [Member] | Other, 1% at December 31, 2014, due 2015 to 2025 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 3.3 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_OnePercentAtDecemberThirtyFirstTwoThousandThirteenOtherLongTermDebtDueTwoThousandFourteenToTwoThousandTwentyFiveMember / us-gaap_LongtermDebtTypeAxis = lnt_OtherLongTermDebtMiscellaneousMember |  | 0.4 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_OnePercentAtDecemberThirtyFirstTwoThousandThirteenOtherLongTermDebtDueTwoThousandFourteenToTwoThousandTwentyFiveMember / us-gaap_LongtermDebtTypeAxis = lnt_OtherLongTermDebtMiscellaneousMember |  |
Interest rate | 1.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_OnePercentAtDecemberThirtyFirstTwoThousandThirteenOtherLongTermDebtDueTwoThousandFourteenToTwoThousandTwentyFiveMember / us-gaap_LongtermDebtTypeAxis = lnt_OtherLongTermDebtMiscellaneousMember |  | 1.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_OnePercentAtDecemberThirtyFirstTwoThousandThirteenOtherLongTermDebtDueTwoThousandFourteenToTwoThousandTwentyFiveMember / us-gaap_LongtermDebtTypeAxis = lnt_OtherLongTermDebtMiscellaneousMember |  |
IPL [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 1,775.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember |  | 1,563.4 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember |  |
Current maturities | (150.0)us-gaap_LongTermDebtCurrent / dei_LegalEntityAxis = lnt_IplMember |  | (38.4)us-gaap_LongTermDebtCurrent / dei_LegalEntityAxis = lnt_IplMember |  |
Unamortized debt (discount) and premium, net | (6.3)us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet / dei_LegalEntityAxis = lnt_IplMember |  | (5.0)us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet / dei_LegalEntityAxis = lnt_IplMember |  |
Long-term debt, net | 1,618.7 us-gaap_LongTermDebtNoncurrent / dei_LegalEntityAxis = lnt_IplMember |  | 1,520.0 us-gaap_LongTermDebtNoncurrent / dei_LegalEntityAxis = lnt_IplMember |  |
Repayments of commercial paper | 60 us-gaap_RepaymentsOfCommercialPaper / dei_LegalEntityAxis = lnt_IplMember |  |  |  |
Payments to retire long-term debt | (38.4)us-gaap_RepaymentsOfLongTermDebt / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_RepaymentsOfLongTermDebt / dei_LegalEntityAxis = lnt_IplMember |  | 0 us-gaap_RepaymentsOfLongTermDebt / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Senior Debentures [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 1,775.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 1,525.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 3.3% senior debenture, due 2015 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 150.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointThreePercentSeniorDebenturesDueTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 150.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointThreePercentSeniorDebenturesDueTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 3.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointThreePercentSeniorDebenturesDueTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 3.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointThreePercentSeniorDebenturesDueTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 5.875% senior debenture, due 2018 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointEightSevenFivePercentSeniorDebenturesDueTwoThousandEighteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointEightSevenFivePercentSeniorDebenturesDueTwoThousandEighteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 5.875%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointEightSevenFivePercentSeniorDebenturesDueTwoThousandEighteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 5.875%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointEightSevenFivePercentSeniorDebenturesDueTwoThousandEighteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 7.25% senior debenture, due 2018 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SevenPointTwoFivePercentSeniorDebenturesDueTwoThousandEighteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SevenPointTwoFivePercentSeniorDebenturesDueTwoThousandEighteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 7.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SevenPointTwoFivePercentSeniorDebenturesDueTwoThousandEighteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 7.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SevenPointTwoFivePercentSeniorDebenturesDueTwoThousandEighteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 3.65% senior debenture, due 2020 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 200.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointSixFivePercentSeniorDebenturesDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 200.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointSixFivePercentSeniorDebenturesDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 3.65%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointSixFivePercentSeniorDebenturesDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 3.65%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointSixFivePercentSeniorDebenturesDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 3.25% senior debenture, due 2024 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember | [1] | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember | [1] |
Interest rate | 3.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_ThreePointTwoFivePercentSeniorDebenturesDueTwoThousandTwentyFourMemberDomain / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |  |  |
IPL [Member] | Senior Debentures [Member] | 5.5% senior debenture, due 2025 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointFivePercentSeniorDebenturesDueTwoThousandTwentyFiveMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 50.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointFivePercentSeniorDebenturesDueTwoThousandTwentyFiveMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 5.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointFivePercentSeniorDebenturesDueTwoThousandTwentyFiveMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 5.50%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointFivePercentSeniorDebenturesDueTwoThousandTwentyFiveMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 6.45% senior debenture, due 2033 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointFourFivePercentSeniorDebenturesDueTwoThousandThirtyThreeMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointFourFivePercentSeniorDebenturesDueTwoThousandThirtyThreeMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 6.45%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointFourFivePercentSeniorDebenturesDueTwoThousandThirtyThreeMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 6.45%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointFourFivePercentSeniorDebenturesDueTwoThousandThirtyThreeMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 6.3% senior debenture, due 2034 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 125.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointThreePercentSeniorDebenturesDueTwoThousandThirtyFourMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 125.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointThreePercentSeniorDebenturesDueTwoThousandThirtyFourMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 6.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointThreePercentSeniorDebenturesDueTwoThousandThirtyFourMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 6.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointThreePercentSeniorDebenturesDueTwoThousandThirtyFourMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 6.25% senior debenture, due 2039 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 300.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentSeniorDebenturesDueTwoThousandThirtyNineMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 300.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentSeniorDebenturesDueTwoThousandThirtyNineMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentSeniorDebenturesDueTwoThousandThirtyNineMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentSeniorDebenturesDueTwoThousandThirtyNineMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Senior Debentures [Member] | 4.7% senior debenture, due 2043 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointSevenPercentSeniorDebenturesDueTwoThousandFortyThreeMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointSevenPercentSeniorDebenturesDueTwoThousandFortyThreeMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
Interest rate | 4.70%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointSevenPercentSeniorDebenturesDueTwoThousandFortyThreeMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  | 4.70%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointSevenPercentSeniorDebenturesDueTwoThousandFortyThreeMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_SeniorDebenturesMember |  |
IPL [Member] | Pollution Control Revenue Bonds [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 38.4 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
IPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember | [4] | 38.4 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember | [4] |
Interest rate |  |  | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenMember / dei_LegalEntityAxis = lnt_IplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
WPL [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 1,580.6 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_WplMember |  | 1,339.1 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_WplMember |  |
Current maturities | (30.6)us-gaap_LongTermDebtCurrent / dei_LegalEntityAxis = lnt_WplMember |  | (8.5)us-gaap_LongTermDebtCurrent / dei_LegalEntityAxis = lnt_WplMember |  |
Unamortized debt (discount) and premium, net | (6.7)us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet / dei_LegalEntityAxis = lnt_WplMember |  | (7.0)us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet / dei_LegalEntityAxis = lnt_WplMember |  |
Long-term debt, net | 1,543.3 us-gaap_LongTermDebtNoncurrent / dei_LegalEntityAxis = lnt_WplMember |  | 1,323.6 us-gaap_LongTermDebtNoncurrent / dei_LegalEntityAxis = lnt_WplMember |  |
WPL [Member] | Debentures [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 1,550.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 1,300.0 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
WPL [Member] | Debentures [Member] | 5% debenture, due 2019 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentDebentureDueTwoThousandNineteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentDebentureDueTwoThousandNineteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentDebentureDueTwoThousandNineteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentDebentureDueTwoThousandNineteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
WPL [Member] | Debentures [Member] | 4.6% debenture, due 2020 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 150.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointSixPercentDebentureDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 150.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointSixPercentDebentureDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 4.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointSixPercentDebentureDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 4.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointSixPercentDebentureDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
WPL [Member] | Debentures [Member] | 2.25% debenture, due 2022 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TwoPointTwoFivePercentDebentureDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TwoPointTwoFivePercentDebentureDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_TwoPointTwoFivePercentDebentureDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 2.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_TwoPointTwoFivePercentDebentureDueTwoThousandTwentyMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
WPL [Member] | Debentures [Member] | 6.25% debenture, due 2034 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentDebentureDueTwoThousandThirtyFourMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 100.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentDebentureDueTwoThousandThirtyFourMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentDebentureDueTwoThousandThirtyFourMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointTwoFivePercentDebentureDueTwoThousandThirtyFourMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
WPL [Member] | Debentures [Member] | 6.375% debenture, due 2037 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 300.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointThreeSevenFivePercentDebentureDueTwoThousandThirtySevenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 300.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SixPointThreeSevenFivePercentDebentureDueTwoThousandThirtySevenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 6.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointThreeSevenFivePercentDebentureDueTwoThousandThirtySevenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 6.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SixPointThreeSevenFivePercentDebentureDueTwoThousandThirtySevenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
WPL [Member] | Debentures [Member] | 7.6% debenture, due 2038 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SevenPointSixPercentDebentureDueTwoThousandThirtyEightMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_SevenPointSixPercentDebentureDueTwoThousandThirtyEightMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
Interest rate | 7.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SevenPointSixPercentDebentureDueTwoThousandThirtyEightMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  | 7.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_SevenPointSixPercentDebentureDueTwoThousandThirtyEightMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |
WPL [Member] | Debentures [Member] | 4.1% debenture, due 2044 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 250.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointOnePercentDebentureDueTwoThousandFortyFourMemberMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember | [2] | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FourPointOnePercentDebentureDueTwoThousandFortyFourMemberMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember | [2] |
Interest rate | 4.10%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FourPointOnePercentDebentureDueTwoThousandFortyFourMemberMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_DebenturesMember |  |  |  |
WPL [Member] | Pollution Control Revenue Bonds [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 30.6 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 39.1 us-gaap_LongTermDebt / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond, due 2015 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 16.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenAndTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember | [3] | 24.5 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenAndTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember | [3] |
Interest rate | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenAndTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenAndTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5.375% pollution control revenue bond, due 2015 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 14.6 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointThreeSevenFivePercentPollutionControlRevenueBondsDueTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 14.6 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePointThreeSevenFivePercentPollutionControlRevenueBondsDueTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
Interest rate | 5.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointThreeSevenFivePercentPollutionControlRevenueBondsDueTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  | 5.375%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_FivePointThreeSevenFivePercentPollutionControlRevenueBondsDueTwoThousandFifteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |
WPL [Member] | Pollution Control Revenue Bonds [Member] | 5% pollution control revenue bond [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Payments to retire long-term debt | (8.5)us-gaap_RepaymentsOfLongTermDebt / us-gaap_DebtInstrumentAxis = lnt_FivePercentPollutionControlRevenueBondsDueTwoThousandFourteenMember / dei_LegalEntityAxis = lnt_WplMember / us-gaap_LongtermDebtTypeAxis = lnt_PollutionControlRevenueBondsMember |  |  |  |
Franklin County Holdings [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2016, 1% at December 31, 2014 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | 60.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwentySixteenMember / dei_LegalEntityAxis = lnt_FranklinCountyHoldingsMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember | [6] | 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwentySixteenMember / dei_LegalEntityAxis = lnt_FranklinCountyHoldingsMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember | [6] |
Interest rate | 1.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwentySixteenMember / dei_LegalEntityAxis = lnt_FranklinCountyHoldingsMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember |  |  |  |
Franklin County Holdings [Member] | Term Loan Credit Agreement [Member] | Term loan credit agreement through 2014, 1% at December 31, 2013 [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Long-term debt issued | $ 0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwoThousandFourteenMember / dei_LegalEntityAxis = lnt_FranklinCountyHoldingsMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember | [6] | $ 60.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwoThousandFourteenMember / dei_LegalEntityAxis = lnt_FranklinCountyHoldingsMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember | [6] |
Interest rate |  |  | 1.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = lnt_TermLoanCreditAgreementThroughTwoThousandFourteenMember / dei_LegalEntityAxis = lnt_FranklinCountyHoldingsMember / us-gaap_LongtermDebtTypeAxis = lnt_TermLoanCreditAgreementMember |  |
[1] | In 2014, IPL issued $250.0 million of 3.25% senior debentures due 2024. The proceeds from the issuance were used by IPL to reduce cash proceeds received from its sales of accounts receivable program, reduce commercial paper classified as long-term debt by $60 million and for general corporate purposes.
[2] | In 2014, WPL issued $250.0 million of 4.1% debentures due 2044. The proceeds from the issuance were used by WPL to reduce commercial paper and for general corporate purposes.
[3] | In 2014, WPL retired $8.5 million of its 5% pollution control revenue bonds.
[4] | In 2014, IPL retired its $38.4 million, 5% pollution control revenue bonds.
[5] | In 2014, Alliant Energy entered into a $250.0 million variable-rate term loan credit agreement and used the proceeds from borrowings under this agreement to retire its $250.0 million, 4% senior notes due 2014.
[6] | In 2014, Franklin County Holdings LLC, Resources’ wholly-owned subsidiary, entered into a $60.0 million variable-rate term loan credit agreement and used the proceeds to retire its borrowings under a term loan credit agreement that matured in December 2014.

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 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 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of debt discount (net of debt premium) that was originally recognized at the issuance of the instrument that has yet to be amortized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28555-108399 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28567-108399 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400 + Details
Name: | us-gaap_DebtInstrumentUnamortizedDiscountPremiumNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + 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 210.5-02.22) -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 210.9-03.16) -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 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_LongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + 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 210.5-02.19,20) -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 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 + Details
Name: | us-gaap_LongTermDebtCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + 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 210.5-02.22) -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 22 -Article 5 + Details
Name: | us-gaap_LongTermDebtNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The cash outflow due to repaying amounts borrowed by issuing commercial paper. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585 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
Name: | us-gaap_RepaymentsOfCommercialPaper
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585 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
Name: | us-gaap_RepaymentsOfLongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | dei_LegalEntityAxis=lnt_FranklinCountyHoldingsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=lnt_DebenturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=lnt_OtherLongTermDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=lnt_OtherLongTermDebtMiscellaneousMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=lnt_PollutionControlRevenueBondsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=lnt_SeniorDebenturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=lnt_SeniorSecuredNotesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=lnt_TermLoanCreditAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |