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Common Equity (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Schedule of Common Equity [Line Items] |  |  |
Shares available for issuance under the OIP, Shareowner Direct Plan and 401(k) Savings Plan | 7,800,000 lnt_SharesAvailableForIssuancePursuantToVariousPlans |  |
Ownership percentage | 15.00%lnt_AmountAcquiredToExerciseShareownerRightsAgreement |  |
Initial right entitlement, number of shares of common stock shareholders have the option to purchase (in shares) | 0.5 lnt_ClassOfWarrantOrRightInitialRightEntitlementNumberOfSharesOfCommonStockShareholdersHaveOptionToPurchase |  |
Exercise price, rights (in dollars per share) | $ 110 us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1 |  |
Percentage of exercise price upon change in beneficial ownership | 200.00%lnt_ClassOfWarrantOrRightExercisePriceOfWarrantOrRightPercentageOfExercisePriceUponChangeInBeneficialOwnership |  |
Other comprehensive income (loss) | $ (0.4)us-gaap_OtherComprehensiveIncomeLossNetOfTax | $ 0.6 us-gaap_OtherComprehensiveIncomeLossNetOfTax | $ 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax
IPL [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Common equity ratio, computation of ratio average, number of months | 13 months |  |
Common equity ratio | 42.00%lnt_CommonEquityRatioRestrictions / dei_LegalEntityAxis = lnt_IplMember |  |
Retained earnings free of dividend restrictions | 538 us-gaap_RetainedEarningsUnappropriated / dei_LegalEntityAxis = lnt_IplMember |  |
Other comprehensive income (loss) | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_IplMember | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_IplMember | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Common equity ratio, computation of ratio average, number of months | 13 months |  |
Retained earnings free of dividend restrictions | 127 us-gaap_RetainedEarningsUnappropriated / dei_LegalEntityAxis = lnt_WplMember |  |
Other comprehensive income (loss) | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_WplMember | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_WplMember | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_WplMember
Minimum [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Ownership percentage | 10.00%lnt_AmountAcquiredToExerciseShareownerRightsAgreement / us-gaap_RangeAxis = us-gaap_MinimumMember |  |
2015 [Member] | WPL [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Common equity ratio | 51.79%lnt_CommonEquityRatioRestrictions / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandFifteenMember |  |
Retained earnings free of dividend restrictions | 127 us-gaap_RetainedEarningsUnappropriated / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandFifteenMember |  |
2016 [Member] | WPL [Member] |  |  |
Schedule of Common Equity [Line Items] |  |  |
Common equity ratio | 52.25%lnt_CommonEquityRatioRestrictions / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandSixteenMember |  |
Retained earnings free of dividend restrictions | $ 135 us-gaap_RetainedEarningsUnappropriated / dei_LegalEntityAxis = lnt_WplMember / us-gaap_PublicUtilitiesRegulatoryProceedingAxis = lnt_TwoThousandSixteenMember |  |

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