XML 178 R106.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Benefit Plans (Defined Benefit Pension And Other Postretirement Benefits Plans) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Benefit Pension Plans [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Service cost | $ 13.1 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | $ 15.7 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | $ 13.5 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Interest cost | 54.1 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 49.0 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 51.6 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Expected return on plan assets | (74.9)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | [1] | (74.0)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | [1] | (68.8)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | [1]
Amortization of prior service cost (credit) | 0 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 0.2 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 0.3 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Amortization of actuarial loss | 19.5 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | [2] | 36.2 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | [2] | 33.3 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | [2]
Additional benefit costs | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 9.0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | [3] | 0.1 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Settlement losses | 0 us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 0 us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 5.4 us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | [4]
Total | 11.8 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 36.1 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |  | 35.4 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Other Postretirement Benefits Plans [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Service cost | 5.2 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 6.3 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 6.9 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
Interest cost | 9.5 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 8.5 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 10.2 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
Expected return on plan assets | (8.3)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [1] | (8.1)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [1] | (7.5)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [1]
Amortization of prior service cost (credit) | (11.9)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [5] | (11.9)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [5] | (12.0)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [5]
Amortization of actuarial loss | 2.4 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [2] | 4.9 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [2] | 6.3 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | [2]
Additional benefit costs | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
Settlement losses | 0 us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 0 us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 0 us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1 / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
Total | (3.1)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | (0.3)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |  | 3.9 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
Cash Balance Plan [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Additional benefit costs |  |  | 9.0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CashBalancePlanMember |  |  |
IPL [Member] | Defined Benefit Pension Plans [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Service cost | 7.2 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 8.6 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 7.5 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
Interest cost | 25.1 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 22.9 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 24.1 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
Expected return on plan assets | (35.7)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1] | (35.2)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1] | (32.6)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1]
Amortization of prior service cost (credit) | 0 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 0.1 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 0.2 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
Amortization of actuarial loss | 8.0 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [2] | 15.2 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [2] | 14.1 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [2]
Additional benefit costs | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 2.6 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [3] | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
Total | 4.6 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 14.2 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 13.3 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
IPL [Member] | Other Postretirement Benefits Plans [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Service cost | 2.4 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 2.9 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 3.0 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
Interest cost | 3.9 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 3.6 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 4.4 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
Expected return on plan assets | (5.8)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1] | (5.6)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1] | (5.1)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [1]
Amortization of prior service cost (credit) | (6.3)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [5] | (6.3)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [5] | (6.3)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [5]
Amortization of actuarial loss | 1.1 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [2] | 2.7 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [2] | 3.5 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember | [2]
Additional benefit costs | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
Total | (4.7)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | (2.7)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |  | (0.5)us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_IplMember |
IPL [Member] | Directly assigned defined benefit pension plans [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Additional benefit costs |  |  | 2.6 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DirectlyAssignedDefinedPensionBenefitsMember / dei_LegalEntityAxis = lnt_IplMember |  |  |
IPL [Member] | Cash Balance Plan [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Additional benefit costs |  |  | 5.5 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CashBalancePlanMember / dei_LegalEntityAxis = lnt_IplMember |  |  |
IPL [Member] | Corporate Service Allocation [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Additional benefit costs |  |  | 2.9 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CorporateServiceAllocationMember / dei_LegalEntityAxis = lnt_IplMember |  |  |
WPL [Member] | Defined Benefit Pension Plans [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Service cost | 4.9 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 5.9 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 5.2 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
Interest cost | 22.6 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 20.7 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 21.6 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
Expected return on plan assets | (32.4)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1] | (31.9)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1] | (29.6)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1]
Amortization of prior service cost (credit) | 0.3 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 0.3 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 0.4 us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
Amortization of actuarial loss | 9.2 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [2] | 17.1 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [2] | 15.7 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [2]
Additional benefit costs | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 0.6 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [3] | 0.1 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
Total | 4.6 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 12.7 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 13.4 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
WPL [Member] | Other Postretirement Benefits Plans [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Service cost | 2.0 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 2.5 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 2.7 us-gaap_DefinedBenefitPlanServiceCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
Interest cost | 3.8 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 3.4 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 4.1 us-gaap_DefinedBenefitPlanInterestCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
Expected return on plan assets | (1.3)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1] | (1.3)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1] | (1.3)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [1]
Amortization of prior service cost (credit) | (3.9)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [5] | (3.9)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [5] | (3.9)us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCredit / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [5]
Amortization of actuarial loss | 1.3 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [2] | 1.9 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [2] | 2.3 us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember | [2]
Additional benefit costs | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 0 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
Total | 1.9 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 2.6 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |  | 3.9 us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = lnt_WplMember |
WPL [Member] | Directly assigned defined benefit pension plans [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Additional benefit costs |  |  | 0.6 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_DirectlyAssignedDefinedPensionBenefitsMember / dei_LegalEntityAxis = lnt_WplMember |  |  |
WPL [Member] | Cash Balance Plan [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Additional benefit costs |  |  | 2.8 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CashBalancePlanMember / dei_LegalEntityAxis = lnt_WplMember |  |  |
WPL [Member] | Corporate Service Allocation [Member] |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |
Additional benefit costs |  |  | $ 2.2 lnt_AggregateAdditionalPaymentsToCertainPlanParticipants / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = lnt_CorporateServiceAllocationMember / dei_LegalEntityAxis = lnt_WplMember |  |  |
[1] | The expected return on plan assets is based on the expected rate of return on plan assets and the fair value approach to the market-related value of plan assets.
[2] | Unrecognized net actuarial gains or losses in excess of 10% of the greater of the plans’ benefit obligations or assets are amortized over the average future service lives of plan participants, except for the Cash Balance Plan where gains or losses outside the 10% threshold are amortized over the time period the participants are expected to receive benefits.
[3] | In 2013, Alliant Energy filed a stipulation agreement with the Court related to the class-action lawsuit against the Cash Balance Plan. As a result, Alliant Energy recognized $9.0 million of additional benefits costs in 2013 related to the agreement. IPL recognized $5.5 million ($2.6 million directly assigned and $2.9 million allocated by Corporate Services) and WPL recognized $2.8 million ($0.6 million directly assigned and $2.2 million allocated by Corporate Services) of additional benefits costs in 2013 related to the agreement.
[4] | Settlement losses related to payments made to retired executives of Alliant Energy.
[5] | Unrecognized prior service costs (credits) for the OPEB plans are amortized over the average future service period to full eligibility of the participants of each plan.

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- Definition The amount of gains or losses recognized in net periodic benefit cost. + 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 (h)(4) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512136 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512171 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time. + 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 (h)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. + 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 (j) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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- Definition Amount of net gain (loss) recognized in net periodic benefit cost as a result of an irrevocable action that relieves the employer (or the plan) of primary responsibility for a benefit obligation and eliminates significant risks related to the obligation and the assets used to effect the settlement. Examples of transactions that constitute a settlement include, but are not limited to, lump-sum cash payments to plan participants in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contracts to cover vested benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 15 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=7577095&loc=d3e8001-114927 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(7) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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