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Common Equity (Narrative) (Details) (USD $) | 3 Months Ended | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Apr. 30, 2015 | Dec. 31, 2015
Common Equity [Line Items] |  |  |  |
Proceeds from issuance of common stock, net | $ 122,100,000 us-gaap_ProceedsFromIssuanceOfCommonStock | $ 0 us-gaap_ProceedsFromIssuanceOfCommonStock |  |
Other comprehensive income | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax |  |
IPL [Member] |  |  |  |
Common Equity [Line Items] |  |  |  |
Retained earnings free of dividend restrictions | 518,000,000 us-gaap_RetainedEarningsUnappropriated / dei_LegalEntityAxis = lnt_IplMember |  |  |
Restricted net assets of subsidiaries | 1,300,000,000 us-gaap_AmountOfRestrictedNetAssetsForConsolidatedAndUnconsolidatedSubsidiaries / dei_LegalEntityAxis = lnt_IplMember |  |  |
Common stock dividends to parent | 35,000,000 us-gaap_DividendsCommonStockCash / dei_LegalEntityAxis = lnt_IplMember |  |  |
Other comprehensive income | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_IplMember | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_IplMember |  |
WPL [Member] |  |  |  |
Common Equity [Line Items] |  |  |  |
Retained earnings free of dividend restrictions | 95,000,000 us-gaap_RetainedEarningsUnappropriated / dei_LegalEntityAxis = lnt_WplMember |  |  |
Restricted net assets of subsidiaries | 1,600,000,000 us-gaap_AmountOfRestrictedNetAssetsForConsolidatedAndUnconsolidatedSubsidiaries / dei_LegalEntityAxis = lnt_WplMember |  |  |
Common stock dividends to parent | 31,800,000 us-gaap_DividendsCommonStockCash / dei_LegalEntityAxis = lnt_WplMember |  |  |
Other comprehensive income | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_WplMember | 0 us-gaap_OtherComprehensiveIncomeLossNetOfTax / dei_LegalEntityAxis = lnt_WplMember |  |
At The Market Offering Program [Member] |  |  |  |
Common Equity [Line Items] |  |  |  |
New shares of common stock issued, At-the-market offering program (in shares) | 1,996,450 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_SubsidiarySaleOfStockAxis = lnt_AtTheMarketOfferingProgramMember |  |  |
Proceeds from issuance of common stock, net | 121,000,000 us-gaap_ProceedsFromIssuanceOfCommonStock / us-gaap_SubsidiarySaleOfStockAxis = lnt_AtTheMarketOfferingProgramMember |  |  |
Fees and commissions from issuance of common stock | 2,000,000 lnt_FeesAndCommissionsFromIssuanceOfCommonStock / us-gaap_SubsidiarySaleOfStockAxis = lnt_AtTheMarketOfferingProgramMember |  |  |
At The Market Offering Program [Member] | Subsequent Event [Member] |  |  |  |
Common Equity [Line Items] |  |  |  |
Maximum aggregate gross sales price of common stock that can be offered and sold |  |  |  | 150,000,000 lnt_MaximumAggregateGrossSalesPriceOfCommonStockThatCanBeOfferedAndSold / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember / us-gaap_SubsidiarySaleOfStockAxis = lnt_AtTheMarketOfferingProgramMember
New shares of common stock issued, At-the-market offering program (in shares) |  |  | 190,167 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember / us-gaap_SubsidiarySaleOfStockAxis = lnt_AtTheMarketOfferingProgramMember |
Proceeds from issuance of common stock, net |  |  | $ 12,000,000 us-gaap_ProceedsFromIssuanceOfCommonStock / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember / us-gaap_SubsidiarySaleOfStockAxis = lnt_AtTheMarketOfferingProgramMember |

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- Definition Amount of restricted net assets of consolidated and unconsolidated subsidiaries as of the end of the most recently completed fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e)(3)(ii)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph e -Subparagraph 3 -Article 4 + Details
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- Definition Amount of paid and unpaid common stock dividends declared with the form of settlement in cash. + 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 S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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