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Consolidated Balance Sheets (Parenthetical) (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Common stock, par value (in dollars per share) | $ 0.01 us-gaap_CommonStockParOrStatedValuePerShare | $ 0.01 us-gaap_CommonStockParOrStatedValuePerShare
Common stock, shares authorized (in shares) | 240,000,000 us-gaap_CommonStockSharesAuthorized | 240,000,000 us-gaap_CommonStockSharesAuthorized
Common stock, shares outstanding (in shares) | 110,935,680 us-gaap_CommonStockSharesOutstanding | 110,943,669 us-gaap_CommonStockSharesOutstanding
Shares in deferred compensation trust (in shares) | 238,935 lnt_CommonStockHeldInTrustShares | 227,469 lnt_CommonStockHeldInTrustShares
Shares in deferred compensation trust, weighted average cost per share (in dollars per share) | $ 37.45 lnt_CommonStockHeldInTrustWeightedAverageValuePerShare | $ 35.25 lnt_CommonStockHeldInTrustWeightedAverageValuePerShare
IPL [Member] |  |
Common stock, par value (in dollars per share) | $ 2.50 us-gaap_CommonStockParOrStatedValuePerShare / dei_LegalEntityAxis = lnt_IplMember | $ 2.50 us-gaap_CommonStockParOrStatedValuePerShare / dei_LegalEntityAxis = lnt_IplMember
Common stock, shares authorized (in shares) | 24,000,000 us-gaap_CommonStockSharesAuthorized / dei_LegalEntityAxis = lnt_IplMember | 24,000,000 us-gaap_CommonStockSharesAuthorized / dei_LegalEntityAxis = lnt_IplMember
Common stock, shares outstanding (in shares) | 13,370,788 us-gaap_CommonStockSharesOutstanding / dei_LegalEntityAxis = lnt_IplMember | 13,370,788 us-gaap_CommonStockSharesOutstanding / dei_LegalEntityAxis = lnt_IplMember
WPL [Member] |  |
Common stock, par value (in dollars per share) | $ 5 us-gaap_CommonStockParOrStatedValuePerShare / dei_LegalEntityAxis = lnt_WplMember | $ 5 us-gaap_CommonStockParOrStatedValuePerShare / dei_LegalEntityAxis = lnt_WplMember
Common stock, shares authorized (in shares) | 18,000,000 us-gaap_CommonStockSharesAuthorized / dei_LegalEntityAxis = lnt_WplMember | 18,000,000 us-gaap_CommonStockSharesAuthorized / dei_LegalEntityAxis = lnt_WplMember
Common stock, shares outstanding (in shares) | 13,236,601 us-gaap_CommonStockSharesOutstanding / dei_LegalEntityAxis = lnt_WplMember | 13,236,601 us-gaap_CommonStockSharesOutstanding / dei_LegalEntityAxis = lnt_WplMember

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- Definition Weighted average value per share; generally not indicative of the fair market value per share. + References No definition available. + Details
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- Definition Face amount or stated value per share of common stock. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition The maximum number of common shares permitted to be issued by an entity's charter and bylaws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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- Definition Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
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