XML 138 R106.htm IDEA: XBRL DOCUMENT
v3.3.1.900

Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
IPL [Member] |  |  |
Change in benefit obligation: |  |  |
Regulatory assets | $ 38.0 | $ 38.2 |
WPL [Member] |  |  |
Change in benefit obligation: |  |  |
Regulatory assets | 29.5 | 28.0 |
Pension Plans, Defined Benefit [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 1,301.5 | 1,113.4 |
Service cost | 15.9 | 13.1 | $ 15.7
Interest cost | 53.6 | 54.1 | 49.0
Plan participants' contributions | 0.0 | 0.0 |
Plan amendments | 0.0 | 0.0 |
Additional benefit costs | 0.5 | 0.0 |
Actuarial (gain) loss | (70.1) | 195.8 |
Gross benefits paid | (95.1) | (74.9) |
Net benefit obligation at December 31 | 1,206.3 | 1,301.5 | 1,113.4
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 1,018.1 | 1,022.9 |
Actual return on plan assets | (30.2) | 66.4 |
Employer contributions | 2.2 | 3.7 |
Plan participants' contributions | 0.0 | 0.0 |
Gross benefits paid | (95.1) | (74.9) |
Fair value of plan assets at December 31 | 895.0 | 1,018.1 | 1,022.9
Under funded status at December 31 | (311.3) | (283.4) |
Pension Plans, Defined Benefit [Member] | IPL [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 603.1 | 514.0 |
Service cost | 8.8 | 7.2 | 8.6
Interest cost | 25.0 | 25.1 | 22.9
Plan participants' contributions | 0.0 | 0.0 |
Plan amendments | 0.0 | 0.0 |
Actuarial (gain) loss | (32.3) | 91.4 |
Gross benefits paid | (48.5) | (34.6) |
Net benefit obligation at December 31 | 556.1 | 603.1 | 514.0
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 484.7 | 485.9 |
Actual return on plan assets | (14.3) | 32.1 |
Employer contributions | 0.8 | 1.3 |
Plan participants' contributions | 0.0 | 0.0 |
Gross benefits paid | (48.5) | (34.6) |
Fair value of plan assets at December 31 | 422.7 | 484.7 | 485.9
Under funded status at December 31 | (133.4) | (118.4) |
Pension Plans, Defined Benefit [Member] | WPL [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 547.6 | 460.8 |
Service cost | 5.8 | 4.9 | 5.9
Interest cost | 22.6 | 22.6 | 20.7
Plan participants' contributions | 0.0 | 0.0 |
Plan amendments | 0.0 | 0.0 |
Additional benefit costs | 0.5 | 0.0 |
Actuarial (gain) loss | (30.0) | 86.7 |
Gross benefits paid | (40.6) | (27.4) |
Net benefit obligation at December 31 | 505.9 | 547.6 | 460.8
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 440.3 | 438.8 |
Actual return on plan assets | (13.0) | 28.6 |
Employer contributions | 0.1 | 0.3 |
Plan participants' contributions | 0.0 | 0.0 |
Gross benefits paid | (40.6) | (27.4) |
Fair value of plan assets at December 31 | 386.8 | 440.3 | 438.8
Under funded status at December 31 | (119.1) | (107.3) |
Other Postretirement Benefits Plans [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 231.1 | 208.7 |
Service cost | 5.5 | 5.2 | 6.3
Interest cost | 9.1 | 9.5 | 8.5
Plan participants' contributions | 3.1 | 2.8 |
Plan amendments | (0.3) | 0.0 |
Additional benefit costs | 0.0 | 0.0 |
Actuarial (gain) loss | (9.4) | 22.3 |
Gross benefits paid | (17.7) | (17.4) |
Net benefit obligation at December 31 | 221.4 | 231.1 | 208.7
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 121.6 | 124.9 |
Actual return on plan assets | (4.9) | 5.6 |
Employer contributions | 4.8 | 5.7 |
Plan participants' contributions | 3.1 | 2.8 |
Gross benefits paid | (17.7) | (17.4) |
Fair value of plan assets at December 31 | 106.9 | 121.6 | 124.9
Under funded status at December 31 | (114.5) | (109.5) |
Other Postretirement Benefits Plans [Member] | IPL [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 96.4 | 87.8 |
Service cost | 2.4 | 2.4 | 2.9
Interest cost | 3.8 | 3.9 | 3.6
Plan participants' contributions | 1.0 | 0.9 |
Plan amendments | (0.1) | 0.0 |
Actuarial (gain) loss | (4.6) | 8.6 |
Gross benefits paid | (7.6) | (7.2) |
Net benefit obligation at December 31 | 91.3 | 96.4 | 87.8
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 78.7 | 81.2 |
Actual return on plan assets | (3.1) | 3.6 |
Employer contributions | 0.2 | 0.2 |
Plan participants' contributions | 1.0 | 0.9 |
Gross benefits paid | (7.6) | (7.2) |
Fair value of plan assets at December 31 | 69.2 | 78.7 | 81.2
Under funded status at December 31 | (22.1) | (17.7) |
Other Postretirement Benefits Plans [Member] | WPL [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 94.0 | 85.6 |
Service cost | 2.1 | 2.0 | 2.5
Interest cost | 3.7 | 3.8 | 3.4
Plan participants' contributions | 1.6 | 1.3 |
Plan amendments | (0.2) | 0.0 |
Additional benefit costs | 0.0 | 0.0 |
Actuarial (gain) loss | (3.5) | 9.2 |
Gross benefits paid | (8.0) | (7.9) |
Net benefit obligation at December 31 | 89.7 | 94.0 | 85.6
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 21.8 | 21.7 |
Actual return on plan assets | (1.1) | 1.2 |
Employer contributions | 4.4 | 5.5 |
Plan participants' contributions | 1.6 | 1.3 |
Gross benefits paid | (8.0) | (7.9) |
Fair value of plan assets at December 31 | 18.7 | 21.8 | $ 21.7
Under funded status at December 31 | $ (71.0) | $ (72.2) |

X
- Definition Defined Benefit Plan, Additional Benefit Costs + References No definition available. + Details
Name: | lnt_DefinedBenefitPlanAdditionalBenefitCosts
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Defined Benefit Plan Regulatory Asset Allocated From Corporate Services + References No definition available. + Details
Name: | lnt_DefinedBenefitPlanRegulatoryAssetAllocatedFromCorporateServices
Namespace Prefix: | lnt_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost) -URI http://asc.fasb.org/extlink&oid=6504192 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6504226 + Details
Name: | us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Gain or Loss -URI http://asc.fasb.org/extlink&oid=6514294 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)(4) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Gain or Loss -URI http://asc.fasb.org/extlink&oid=6749293 + Details
Name: | us-gaap_DefinedBenefitPlanActuarialGainLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition 1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular 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 (a) -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 -Glossary Projected Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6522206 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Postretirement Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503904 + Details
Name: | us-gaap_DefinedBenefitPlanBenefitObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Benefits -URI http://asc.fasb.org/extlink&oid=6506267 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Benefits -URI http://asc.fasb.org/extlink&oid=6506309 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(6) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Postretirement Benefits -URI http://asc.fasb.org/extlink&oid=6521376 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(5) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanBenefitsPaid
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanChangeInBenefitObligationRollForward
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanChangeInFairValueOfPlanAssetsRollForward
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The increase in the fair value of plan assets from contributions made by the employer. + 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 (b)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount of contributions made by plan participants. This item represents a periodic increase to the plan obligation and an increase to plan assets. + 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 (b)(4) -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)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanContributionsByPlanParticipants
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total. + 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 (c) -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 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=21915240&loc=d3e1703-114919 + Details
Name: | us-gaap_DefinedBenefitPlanFundedStatusOfPlan
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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
Name: | us-gaap_DefinedBenefitPlanInterestCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount of increase or decrease due to a change in the terms of an existing plan or the initiation of a new plan. A plan amendment may increase or decrease benefits, including those attributed to years of service already rendered. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Plan Amendment -URI http://asc.fasb.org/extlink&oid=6520743 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)(7) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Plan Amendment -URI http://asc.fasb.org/extlink&oid=6520782 + Details
Name: | us-gaap_DefinedBenefitPlanPlanAmendments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Service Cost (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6525008 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)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanServiceCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |