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Benefit Plans (Expected Benefit Payments) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | $ 84.3 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
Expected benefit payments, 2016 | 88.3 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo
Expected benefit payments, 2017 | 86.0 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree
Expected benefit payments, 2018 | 89.8 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour
Expected benefit payments, 2019 | 90.6 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive
Expected benefit payments, 2020 - 2024 | 480.1 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter
Defined Benefit Plans Qualified And Non Qualified Pension Benefits [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | 66.7 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember
Expected benefit payments, 2016 | 71.5 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember
Expected benefit payments, 2017 | 69.3 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember
Expected benefit payments, 2018 | 72.9 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember
Expected benefit payments, 2019 | 73.6 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember
Expected benefit payments, 2020 - 2024 | 396.1 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember
Other Postretirement Benefits Plans [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | 17.6 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Expected benefit payments, 2016 | 16.8 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Expected benefit payments, 2017 | 16.7 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Expected benefit payments, 2018 | 16.9 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Expected benefit payments, 2019 | 17.0 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Expected benefit payments, 2020 - 2024 | 84.0 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
IPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | 37.8 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2016 | 38.8 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2017 | 40.3 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2018 | 42.1 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2019 | 41.9 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2020 - 2024 | 225.2 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Defined Benefit Plans Qualified And Non Qualified Pension Benefits [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | 30.4 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2016 | 31.6 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2017 | 33.1 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2018 | 34.9 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2019 | 34.7 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2020 - 2024 | 189.9 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_IplMember
IPL [Member] | Other Postretirement Benefits Plans [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | 7.4 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2016 | 7.2 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2017 | 7.2 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2018 | 7.2 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2019 | 7.2 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember
Expected benefit payments, 2020 - 2024 | 35.3 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | 36.1 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2016 | 35.0 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2017 | 35.8 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2018 | 37.1 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2019 | 37.6 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2020 - 2024 | 197.6 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Defined Benefit Plans Qualified And Non Qualified Pension Benefits [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | 28.4 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2016 | 28.0 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2017 | 28.8 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2018 | 30.1 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2019 | 30.5 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2020 - 2024 | 163.4 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DefinedBenefitPlansQualifiedAndNonQualifiedPensionBenefitsMember / dei_LegalEntityAxis = lnt_WplMember
WPL [Member] | Other Postretirement Benefits Plans [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Expected benefit payments, 2015 | 7.7 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2016 | 7.0 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearTwo / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2017 | 7.0 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearThree / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2018 | 7.0 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFour / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2019 | 7.1 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsYearFive / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember
Expected benefit payments, 2020 - 2024 | $ 34.2 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsFiveFiscalYearsThereafter / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember

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- Definition Amount of benefits from a defined benefit plan expected to be paid in the five fiscal years after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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