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Related Parties (Related Amounts Billed Between Parties) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
ATC Billings To WPL [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Amounts billed between related parties | $ 96 us-gaap_RelatedPartyTransactionAmountsOfTransaction / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_AtcBillingsToWplMember | $ 96 us-gaap_RelatedPartyTransactionAmountsOfTransaction / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_AtcBillingsToWplMember | $ 90 us-gaap_RelatedPartyTransactionAmountsOfTransaction / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_AtcBillingsToWplMember
WPL Billings To ATC [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Amounts billed between related parties | $ 9 us-gaap_RelatedPartyTransactionAmountsOfTransaction / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_WplBillingsToAtcMember | $ 12 us-gaap_RelatedPartyTransactionAmountsOfTransaction / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_WplBillingsToAtcMember | $ 11 us-gaap_RelatedPartyTransactionAmountsOfTransaction / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = lnt_WplBillingsToAtcMember

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- Definition Amount of transactions with related party during the financial reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39622-107864 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39603-107864 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 850 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864 + Details
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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
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Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=lnt_WplBillingsToAtcMember
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Data Type: | na
Balance Type: |
Period Type: |