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Investments (Unconsolidated Equity Investments) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Carrying value | $ 296.6 us-gaap_EquityMethodInvestments |  | $ 281.4 us-gaap_EquityMethodInvestments |  | $ 266.6 us-gaap_EquityMethodInvestments |
Equity (income) / loss | (40.4)us-gaap_IncomeLossFromEquityMethodInvestments |  | (43.7)us-gaap_IncomeLossFromEquityMethodInvestments |  | (41.3)us-gaap_IncomeLossFromEquityMethodInvestments |
WPL [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Equity (income) / loss | (42.8)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember |  | (43.7)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember |  | (42.1)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember |
ATC [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest | 16.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] |  |  |  |
Carrying value | 286.5 us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] | 272.1 us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] |  |
Equity (income) / loss | (41.9)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] | (42.7)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] | (41.3)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1]
ATC [Member] | WPL [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest | 16.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] |  |  |  |
Carrying value | 286.5 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] | 272.1 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] |  |
Equity (income) / loss | (41.9)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] | (42.7)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1] | (41.3)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_AmericanTransmissionCompanyLlcAtcMember | [1]
Wisconsin River Power Company [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest | 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  |  |  |  |
Carrying value | 7.8 us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  | 7.0 us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  |  |
Equity (income) / loss | (0.9)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  | (1.0)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  | (0.8)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |
Wisconsin River Power Company [Member] | WPL [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership interest | 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  |  |  |  |
Carrying value | 7.8 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  | 7.0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  |  |
Equity (income) / loss | (0.9)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  | (1.0)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |  | (0.8)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_WisconsinRiverPowerCompanyMember |
Other [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Carrying value | 2.3 us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember |  | 2.3 us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember |  |  |
Equity (income) / loss | 2.4 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember |  | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember |  | 0.8 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_OtherEquityMethodInvestmentsMember |
Totals [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Carrying value | 296.6 us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |  | 281.4 us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |  |  |
Equity (income) / loss | (40.4)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |  | (43.7)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |  | (41.3)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |
Totals [Member] | WPL [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Carrying value | 294.3 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |  | 279.1 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |  |  |
Equity (income) / loss | $ (42.8)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |  | $ (43.7)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |  | $ (42.1)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = lnt_TotalsMember |
[1] | Alliant Energy and WPL have the ability to exercise significant influence over ATC’s financial and operating policies through their participation on ATC’s Board of Directors. Refer to Note 18 for information regarding related party transactions with ATC.

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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 + Details
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