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Benefit Plans (Defined Contribution Retirement Plans) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
401(k) costs | $ 6.7 us-gaap_DefinedContributionPlanCostRecognized |  | $ 5.9 us-gaap_DefinedContributionPlanCostRecognized |
IPL [Member] |  |  |  |
401(k) costs | 3.4 us-gaap_DefinedContributionPlanCostRecognized / dei_LegalEntityAxis = lnt_IplMember | [1] | 3.1 us-gaap_DefinedContributionPlanCostRecognized / dei_LegalEntityAxis = lnt_IplMember | [1]
WPL [Member] |  |  |  |
401(k) costs | $ 2.9 us-gaap_DefinedContributionPlanCostRecognized / dei_LegalEntityAxis = lnt_WplMember | [1] | $ 2.6 us-gaap_DefinedContributionPlanCostRecognized / dei_LegalEntityAxis = lnt_WplMember | [1]
[1] | IPL’s and WPL’s amounts include allocated costs associated with Corporate Services employees.

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
Name: | us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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