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Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Schedule of Investments [Line Items] |  |
Equity (income) loss from unconsolidated investments, net | $ (6.5)us-gaap_IncomeLossFromEquityMethodInvestments | $ (11.4)us-gaap_IncomeLossFromEquityMethodInvestments
WPL [Member] |  |
Schedule of Investments [Line Items] |  |
Equity (income) loss from unconsolidated investments, net | (7.8)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember | (11.4)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember
ATC [Member] |  |
Schedule of Investments [Line Items] |  |
Equity (income) loss from unconsolidated investments, net | (7.8)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | (11.2)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember
ATC [Member] | WPL [Member] |  |
Schedule of Investments [Line Items] |  |
Equity (income) loss from unconsolidated investments, net | (7.8)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | (11.2)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember
Other [Member] |  |
Schedule of Investments [Line Items] |  |
Equity (income) loss from unconsolidated investments, net | 1.3 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember | (0.2)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember
Other [Member] | WPL [Member] |  |
Schedule of Investments [Line Items] |  |
Equity (income) loss from unconsolidated investments, net | $ 0 us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember | $ (0.2)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember

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- Definition This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 9 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.12) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 11 -Article 7 + Details
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