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v2.4.1.9

Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 804.1 us-gaap_Revenues | $ 843.1 us-gaap_Revenues | $ 750.3 us-gaap_Revenues | $ 952.8 us-gaap_Revenues | $ 832.6 us-gaap_Revenues | $ 866.6 us-gaap_Revenues | $ 718.0 us-gaap_Revenues | $ 859.6 us-gaap_Revenues | $ 3,350.3 us-gaap_Revenues | $ 3,276.8 us-gaap_Revenues | $ 3,094.5 us-gaap_Revenues
Depreciation and amortization |  |  |  |  |  |  |  |  | 388.1 us-gaap_DepreciationAndAmortization | 370.9 us-gaap_DepreciationAndAmortization | 332.4 us-gaap_DepreciationAndAmortization
Operating income (loss) | 91.3 us-gaap_OperatingIncomeLoss | 194.8 us-gaap_OperatingIncomeLoss | 103.3 us-gaap_OperatingIncomeLoss | 154.2 us-gaap_OperatingIncomeLoss | 108.6 us-gaap_OperatingIncomeLoss | 201.4 us-gaap_OperatingIncomeLoss | 103.2 us-gaap_OperatingIncomeLoss | 120.7 us-gaap_OperatingIncomeLoss | 543.6 us-gaap_OperatingIncomeLoss | 533.9 us-gaap_OperatingIncomeLoss | 519.7 us-gaap_OperatingIncomeLoss
Interest expense |  |  |  |  |  |  |  |  | 180.6 us-gaap_InterestExpense | 172.8 us-gaap_InterestExpense | 156.7 us-gaap_InterestExpense
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | (40.4)us-gaap_IncomeLossFromEquityMethodInvestments | (43.7)us-gaap_IncomeLossFromEquityMethodInvestments | (41.3)us-gaap_IncomeLossFromEquityMethodInvestments
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 44.3 us-gaap_IncomeTaxExpenseBenefit | 53.9 us-gaap_IncomeTaxExpenseBenefit | 89.4 us-gaap_IncomeTaxExpenseBenefit
Net income (loss) attributable to common shareowners | 60.0 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 153.3 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 61.8 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 108.0 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 65.5 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 157.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 65.3 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 69.9 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 383.1 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 358.3 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 319.8 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Total assets | 12,085.9 us-gaap_Assets |  |  |  | 11,112.4 us-gaap_Assets |  |  |  | 12,085.9 us-gaap_Assets | 11,112.4 us-gaap_Assets | 10,785.5 us-gaap_Assets
Investments in equity method subsidiaries | 296.6 us-gaap_EquityMethodInvestments |  |  |  | 281.4 us-gaap_EquityMethodInvestments |  |  |  | 296.6 us-gaap_EquityMethodInvestments | 281.4 us-gaap_EquityMethodInvestments | 266.6 us-gaap_EquityMethodInvestments
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 902.8 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment | 798.3 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment | 1,158.1 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment
IPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | 431.1 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 476.2 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 411.9 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 528.9 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 463.1 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 494.4 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 383.4 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 477.9 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 1,848.1 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 1,818.8 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember | 1,650.3 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 197.5 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember | 191.1 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember | 188.9 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember
Operating income (loss) | 23.8 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 93.9 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 34.0 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 57.5 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 36.2 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 100.0 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 34.7 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 41.1 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 209.2 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 212.0 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember | 200.3 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember
Interest expense |  |  |  |  |  |  |  |  | 89.9 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_IplMember | 81.3 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_IplMember | 78.5 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_IplMember
Income tax expense (benefit) |  |  |  |  |  |  |  |  | (51.7)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | (37.9)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember | (19.8)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember
Net income (loss) attributable to common shareowners | 20.1 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 102.5 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 18.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 43.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 18.5 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 110.0 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 22.2 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 22.9 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 184.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 173.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember | 137.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember
Total assets | 6,461.8 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember |  |  |  | 5,806.0 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember |  |  |  | 6,461.8 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember | 5,806.0 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember |
WPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues | 359.8 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 354.4 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 324.5 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 410.4 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 355.7 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 360.9 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 319.9 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 369.8 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 1,449.1 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 1,406.3 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember | 1,392.0 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 181.2 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember | 172.2 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember | 140.9 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember
Operating income (loss) | 59.4 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 93.9 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 60.0 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 87.7 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 67.8 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 95.9 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 59.7 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 72.7 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 301.0 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 296.1 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember | 284.8 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember
Interest expense |  |  |  |  |  |  |  |  | 86.4 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_WplMember | 85.0 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_WplMember | 80.2 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_WplMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | (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
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 85.6 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | 87.2 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember | 94.6 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember
Net income (loss) attributable to common shareowners | 29.1 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 61.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 34.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 54.8 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 38.2 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 61.3 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 34.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 42.0 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 180.1 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 175.9 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember | 162.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember
Total assets | 5,128.2 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember |  |  |  | 4,804.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember |  |  |  | 5,128.2 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember | 4,804.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember |
Electric [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 2,713.6 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 2,689.0 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 2,589.3 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 347.0 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 333.0 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 299.3 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Operating income (loss) |  |  |  |  |  |  |  |  | 442.4 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 444.5 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 426.2 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | (42.8)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | (43.7)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | (42.1)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Total assets | 9,660.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 9,018.6 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 9,660.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 9,018.6 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 8,438.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Investments in equity method subsidiaries | 294.3 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 279.1 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 294.3 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 279.1 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 264.3 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 774.8 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 677.3 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 994.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Electric [Member] | IPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,493.3 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 1,491.8 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 1,371.1 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 178.7 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 173.1 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 171.2 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Operating income (loss) |  |  |  |  |  |  |  |  | 166.8 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 173.1 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 166.2 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Total assets | 5,398.3 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 4,905.3 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 5,398.3 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 4,905.3 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 4,500.9 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 490.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 365.4 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 291.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Electric [Member] | WPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,220.3 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 1,197.2 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 1,218.2 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 168.3 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 159.9 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 128.1 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Operating income (loss) |  |  |  |  |  |  |  |  | 275.6 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 271.4 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 260.0 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | (42.8)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | (43.7)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | (42.1)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Total assets | 4,262.1 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 4,113.3 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 4,262.1 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 4,113.3 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 3,937.9 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Investments in equity method subsidiaries | 294.3 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 279.1 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember |  |  |  | 294.3 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 279.1 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 264.3 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 284.8 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 311.9 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember | 703.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_ElectricMember
Gas [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 517.5 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 464.8 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 396.3 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 29.9 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 28.8 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 29.1 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Operating income (loss) |  |  |  |  |  |  |  |  | 53.8 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 57.3 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 51.5 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Total assets | 913.5 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 859.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 913.5 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 859.3 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 814.8 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Investments in equity method subsidiaries | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 63.2 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 47.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 31.4 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Gas [Member] | IPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 296.5 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 273.9 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 226.7 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 17.0 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 16.5 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 16.3 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Operating income (loss) |  |  |  |  |  |  |  |  | 25.7 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 29.8 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 24.2 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Total assets | 544.1 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 518.8 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 544.1 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 518.8 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 479.5 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 35.1 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 27.5 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 16.4 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Gas [Member] | WPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 221.0 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 190.9 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 169.6 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 12.9 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 12.3 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 12.8 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Operating income (loss) |  |  |  |  |  |  |  |  | 28.1 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 27.5 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 27.3 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | 0 us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Total assets | 369.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 340.5 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 369.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 340.5 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 335.3 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Investments in equity method subsidiaries | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember |  |  |  | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 28.1 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 19.5 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember | 15.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_GasMember
Other Utility [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 66.1 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 71.3 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 56.7 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 1.8 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 1.5 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 1.4 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Operating income (loss) |  |  |  |  |  |  |  |  | 14.0 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 6.3 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 7.4 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Total assets | 1,016.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 732.5 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 1,016.1 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 732.5 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 966.0 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Investments in equity method subsidiaries | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 0.9 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 7.3 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0.1 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Other Utility [Member] | IPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 58.3 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 53.1 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 52.5 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 1.8 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 1.5 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 1.4 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Operating income (loss) |  |  |  |  |  |  |  |  | 16.7 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 9.1 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 9.9 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Total assets | 519.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 381.9 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 519.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 381.9 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 476.6 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 0.9 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 7.3 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0.1 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Other Utility [Member] | WPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 7.8 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 18.2 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 4.2 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 0 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Operating income (loss) |  |  |  |  |  |  |  |  | (2.7)us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | (2.8)us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | (2.5)us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | 0 us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Total assets | 496.7 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 350.6 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 496.7 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 350.6 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 489.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Investments in equity method subsidiaries | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember |  |  |  | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember | 0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_OtherUtilityMember
Utility Business [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 3,297.2 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 3,225.1 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 3,042.3 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 378.7 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 363.3 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 329.8 us-gaap_DepreciationAndAmortization / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Operating income (loss) |  |  |  |  |  |  |  |  | 510.2 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 508.1 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 485.1 us-gaap_OperatingIncomeLoss / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Interest expense |  |  |  |  |  |  |  |  | 176.3 us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 166.3 us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 158.7 us-gaap_InterestExpense / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | (42.8)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | (43.7)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | (42.1)us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 33.9 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 49.3 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 74.8 us-gaap_IncomeTaxExpenseBenefit / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Net income (loss) attributable to common shareowners |  |  |  |  |  |  |  |  | 364.5 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 349.5 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 300.0 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Total assets | 11,590.0 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 10,610.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 11,590.0 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 10,610.4 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 10,219.6 us-gaap_Assets / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Investments in equity method subsidiaries | 294.3 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 279.1 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 294.3 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 279.1 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 264.3 us-gaap_EquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 838.9 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 731.6 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 1,025.5 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Utility Business [Member] | IPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,848.1 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 1,818.8 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 1,650.3 us-gaap_Revenues / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 197.5 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 191.1 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 188.9 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Operating income (loss) |  |  |  |  |  |  |  |  | 209.2 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 212.0 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 200.3 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Interest expense |  |  |  |  |  |  |  |  | 89.9 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 81.3 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 78.5 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Income tax expense (benefit) |  |  |  |  |  |  |  |  | (51.7)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | (37.9)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | (19.8)us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Net income (loss) attributable to common shareowners |  |  |  |  |  |  |  |  | 184.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 173.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 137.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Total assets | 6,461.8 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 5,806.0 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 6,461.8 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 5,806.0 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 5,457.0 us-gaap_Assets / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 526.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 400.2 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 307.5 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_IplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Utility Business [Member] | WPL [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 1,449.1 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 1,406.3 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 1,392.0 us-gaap_Revenues / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 181.2 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 172.2 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 140.9 us-gaap_DepreciationAndAmortization / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Operating income (loss) |  |  |  |  |  |  |  |  | 301.0 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 296.1 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 284.8 us-gaap_OperatingIncomeLoss / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Interest expense |  |  |  |  |  |  |  |  | 86.4 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 85.0 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 80.2 us-gaap_InterestExpense / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | (42.8)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | (43.7)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | (42.1)us-gaap_IncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 85.6 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 87.2 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 94.6 us-gaap_IncomeTaxExpenseBenefit / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Net income (loss) attributable to common shareowners |  |  |  |  |  |  |  |  | 180.1 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 175.9 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 162.4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Total assets | 5,128.2 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 4,804.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 5,128.2 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 4,804.4 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 4,762.6 us-gaap_Assets / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Investments in equity method subsidiaries | 294.3 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 279.1 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember |  |  |  | 294.3 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 279.1 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 264.3 us-gaap_EquityMethodInvestments / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | 312.9 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 331.4 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember | 718.0 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / dei_LegalEntityAxis = lnt_WplMember / us-gaap_StatementBusinessSegmentsAxis = lnt_UtilityBusinessMember
Non-Regulated [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  |  | 53.1 us-gaap_Revenues / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 51.7 us-gaap_Revenues / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 52.2 us-gaap_Revenues / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Depreciation and amortization |  |  |  |  |  |  |  |  | 9.4 us-gaap_DepreciationAndAmortization / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 7.6 us-gaap_DepreciationAndAmortization / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 2.6 us-gaap_DepreciationAndAmortization / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Operating income (loss) |  |  |  |  |  |  |  |  | 33.4 us-gaap_OperatingIncomeLoss / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 25.8 us-gaap_OperatingIncomeLoss / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 34.6 us-gaap_OperatingIncomeLoss / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Interest expense |  |  |  |  |  |  |  |  | 4.3 us-gaap_InterestExpense / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 6.5 us-gaap_InterestExpense / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | (2.0)us-gaap_InterestExpense / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Equity (income) loss from unconsolidated investments, net |  |  |  |  |  |  |  |  | 2.4 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 0 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 0.8 us-gaap_IncomeLossFromEquityMethodInvestments / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Income tax expense (benefit) |  |  |  |  |  |  |  |  | 10.4 us-gaap_IncomeTaxExpenseBenefit / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 4.6 us-gaap_IncomeTaxExpenseBenefit / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 14.6 us-gaap_IncomeTaxExpenseBenefit / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Net income (loss) attributable to common shareowners |  |  |  |  |  |  |  |  | 18.6 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 8.8 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 19.8 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Total assets | 495.9 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  |  |  | 502.0 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  |  |  | 495.9 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 502.0 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 565.9 us-gaap_Assets / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Investments in equity method subsidiaries | 2.3 us-gaap_EquityMethodInvestments / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  |  |  | 2.3 us-gaap_EquityMethodInvestments / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember |  |  |  | 2.3 us-gaap_EquityMethodInvestments / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 2.3 us-gaap_EquityMethodInvestments / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | 2.3 us-gaap_EquityMethodInvestments / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember
Construction and acquisition expenditures |  |  |  |  |  |  |  |  | $ 63.9 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | $ 66.7 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember | $ 132.6 us-gaap_PaymentsToAcquirePropertyPlantAndEquipment / us-gaap_PublicUtilityRegulatedOrUnregulatedStatusAxis = us-gaap_UnregulatedOperationMember

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