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Asset Retirement Obligations (Reconciliation Of Changes In Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Balance, January 1 | $ 114.0 us-gaap_AssetRetirementObligation | $ 109.7 us-gaap_AssetRetirementObligation
Liabilities settled | (2.2)us-gaap_AssetRetirementObligationLiabilitiesSettled | (0.5)us-gaap_AssetRetirementObligationLiabilitiesSettled
Liabilities incurred | 0 us-gaap_AssetRetirementObligationLiabilitiesIncurred | 16.5 us-gaap_AssetRetirementObligationLiabilitiesIncurred
Accretion expense | 1.0 us-gaap_AssetRetirementObligationAccretionExpense | 1.0 us-gaap_AssetRetirementObligationAccretionExpense
Balance, March 31 | 112.8 us-gaap_AssetRetirementObligation | 126.7 us-gaap_AssetRetirementObligation
IPL [Member] |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Balance, January 1 | 51.8 us-gaap_AssetRetirementObligation / dei_LegalEntityAxis = lnt_IplMember | 47.9 us-gaap_AssetRetirementObligation / dei_LegalEntityAxis = lnt_IplMember
Liabilities settled | (2.2)us-gaap_AssetRetirementObligationLiabilitiesSettled / dei_LegalEntityAxis = lnt_IplMember | (0.3)us-gaap_AssetRetirementObligationLiabilitiesSettled / dei_LegalEntityAxis = lnt_IplMember
Liabilities incurred | 0 us-gaap_AssetRetirementObligationLiabilitiesIncurred / dei_LegalEntityAxis = lnt_IplMember | 16.3 us-gaap_AssetRetirementObligationLiabilitiesIncurred / dei_LegalEntityAxis = lnt_IplMember
Accretion expense | 0.4 us-gaap_AssetRetirementObligationAccretionExpense / dei_LegalEntityAxis = lnt_IplMember | 0.5 us-gaap_AssetRetirementObligationAccretionExpense / dei_LegalEntityAxis = lnt_IplMember
Balance, March 31 | 50.0 us-gaap_AssetRetirementObligation / dei_LegalEntityAxis = lnt_IplMember | 64.4 us-gaap_AssetRetirementObligation / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] |  |
Balance, January 1 | 52.4 us-gaap_AssetRetirementObligation / dei_LegalEntityAxis = lnt_WplMember | 52.4 us-gaap_AssetRetirementObligation / dei_LegalEntityAxis = lnt_WplMember
Liabilities settled | 0 us-gaap_AssetRetirementObligationLiabilitiesSettled / dei_LegalEntityAxis = lnt_WplMember | (0.2)us-gaap_AssetRetirementObligationLiabilitiesSettled / dei_LegalEntityAxis = lnt_WplMember
Liabilities incurred | 0 us-gaap_AssetRetirementObligationLiabilitiesIncurred / dei_LegalEntityAxis = lnt_WplMember | 0.2 us-gaap_AssetRetirementObligationLiabilitiesIncurred / dei_LegalEntityAxis = lnt_WplMember
Accretion expense | 0.4 us-gaap_AssetRetirementObligationAccretionExpense / dei_LegalEntityAxis = lnt_WplMember | 0.4 us-gaap_AssetRetirementObligationAccretionExpense / dei_LegalEntityAxis = lnt_WplMember
Balance, March 31 | $ 52.8 us-gaap_AssetRetirementObligation / dei_LegalEntityAxis = lnt_WplMember | $ 52.8 us-gaap_AssetRetirementObligation / dei_LegalEntityAxis = lnt_WplMember

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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Asset Retirement Obligation -URI http://asc.fasb.org/extlink&oid=6505190 + Details
Name: | us-gaap_AssetRetirementObligation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
Name: | us-gaap_AssetRetirementObligationAccretionExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of asset retirement obligations incurred during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
Name: | us-gaap_AssetRetirementObligationLiabilitiesSettled
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=lnt_IplMember
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Data Type: | na
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- Details
Name: | dei_LegalEntityAxis=lnt_WplMember
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Data Type: | na
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