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Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Segment Reporting Information [Line Items]                      
Operating revenues $ 832.6 $ 866.6 $ 718.0 $ 859.6 $ 750.9 $ 887.6 $ 690.3 $ 765.7 $ 3,276.8 $ 3,094.5 $ 3,221.4
Depreciation and amortization                 370.9 332.4 321.0
Operating income (loss) 108.6 201.4 103.2 120.7 101.6 213.7 108.8 95.6 533.9 519.7 513.3
Interest expense, net of AFUDC                 142.0 134.8 146.3
Equity (income) loss from unconsolidated investments, net                 (43.7) (41.3) (39.3)
Interest income and other                 (0.4) (4.0) (4.3)
Income tax expense (benefit)                 53.9 89.4 69.2
Income from continuing operations, net of tax                 382.1 340.8 341.4
Loss from discontinued operations, net of tax                 (5.9) (5.1) (19.5)
Net income                 376.2 335.7 321.9
Preferred dividends                 17.9 15.9 18.3
Net income attributable to Alliant Energy common shareowners 65.5 157.6 65.3 69.9 68.3 150.7 65.9 34.9 358.3 319.8 303.6
Total assets 11,112.4       10,785.5       11,112.4 10,785.5 9,687.9
Investments in equity method subsidiaries 281.4       266.6       281.4 266.6 249.6
Construction and acquisition expenditures                 798.3 1,158.1 673.4
IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues 463.1 494.4 383.4 477.9 393.2 497.7 360.7 398.7 1,818.8 1,650.3 1,740.1
Depreciation and amortization                 191.1 188.9 179.1
Operating income (loss) 36.2 100.0 34.7 41.1 29.0 112.0 35.4 23.9 212.0 200.3 208.4
Interest income and other                 (0.3) (0.2) (0.2)
Income tax expense (benefit)                 (37.9) (19.8) (3.6)
Net income 21.1 112.6 24.7 31.5 25.6 106.5 19.6 (1.5) 189.9 150.2 139.3
Preferred dividends                 16.3 12.6 15.0
Net income attributable to Alliant Energy common shareowners 18.5 110.0 22.2 22.9 22.4 103.3 16.6 (4.7) 173.6 137.6 124.3
Total assets 5,806.0       5,457.0       5,806.0 5,457.0  
WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues 355.7 360.9 319.9 369.8 345.2 376.6 315.7 354.5 1,406.3 1,392.0 1,434.4
Depreciation and amortization                 172.2 140.9 140.1
Operating income (loss) 67.8 95.9 59.7 72.7 65.3 93.4 61.5 64.6 296.1 284.8 280.4
Equity (income) loss from unconsolidated investments, net                 (43.7) (42.1) (38.7)
Interest income and other                 (0.1) (0.1) 0
Income tax expense (benefit)                 87.2 94.6 81.9
Net income 38.2 61.3 34.4 43.6 41.0 56.7 36.1 31.9 177.5 165.7 163.5
Preferred dividends                 1.6 3.3 3.3
Net income attributable to Alliant Energy common shareowners 38.2 61.3 34.4 42.0 40.2 55.9 35.2 31.1 175.9 162.4 160.2
Total assets 4,804.4       4,762.6       4,804.4 4,762.6  
Electric [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 2,689.0 2,589.3 2,635.8
Depreciation and amortization                 333.0 299.3 289.0
Operating income (loss)                 444.5 426.2 444.2
Equity (income) loss from unconsolidated investments, net                 (43.7) (42.1) (38.7)
Total assets 9,018.6       8,438.8       9,018.6 8,438.8 7,524.5
Investments in equity method subsidiaries 279.1       264.3       279.1 264.3 246.5
Construction and acquisition expenditures                 677.3 994.0 542.7
Electric [Member] | IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 1,491.8 1,371.1 1,408.3
Depreciation and amortization                 173.1 171.2 161.3
Operating income (loss)                 173.1 166.2 181.6
Total assets 4,905.3       4,500.9       4,905.3 4,500.9 4,208.2
Construction and acquisition expenditures                 365.4 291.0 245.6
Electric [Member] | WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 1,197.2 1,218.2 1,227.5
Depreciation and amortization                 159.9 128.1 127.7
Operating income (loss)                 271.4 260.0 262.6
Equity (income) loss from unconsolidated investments, net                 (43.7) (42.1) (38.7)
Total assets 4,113.3       3,937.9       4,113.3 3,937.9 3,316.3
Investments in equity method subsidiaries 279.1       264.3       279.1 264.3 246.5
Construction and acquisition expenditures                 311.9 703.0 297.1
Gas [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 464.8 396.3 476.7
Depreciation and amortization                 28.8 29.1 28.4
Operating income (loss)                 57.3 51.5 47.8
Equity (income) loss from unconsolidated investments, net                 0 0 0
Total assets 859.3       814.8       859.3 814.8 831.9
Investments in equity method subsidiaries 0       0       0 0 0
Construction and acquisition expenditures                 47.0 31.4 38.0
Gas [Member] | IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 273.9 226.7 276.3
Depreciation and amortization                 16.5 16.3 16.0
Operating income (loss)                 29.8 24.2 20.6
Total assets 518.8       479.5       518.8 479.5 471.1
Construction and acquisition expenditures                 27.5 16.4 21.2
Gas [Member] | WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 190.9 169.6 200.4
Depreciation and amortization                 12.3 12.8 12.4
Operating income (loss)                 27.5 27.3 27.2
Equity (income) loss from unconsolidated investments, net                 0 0 0
Total assets 340.5       335.3       340.5 335.3 360.8
Investments in equity method subsidiaries 0       0       0 0 0
Construction and acquisition expenditures                 19.5 15.0 16.8
Other Utility [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 71.3 56.7 62.0
Depreciation and amortization                 1.5 1.4 1.8
Operating income (loss)                 6.3 7.4 (3.2)
Equity (income) loss from unconsolidated investments, net                 0 0 0
Total assets 732.5       966.0       732.5 966.0 781.1
Investments in equity method subsidiaries 0       0       0 0 0
Construction and acquisition expenditures                 7.3 0.1 27.4
Other Utility [Member] | IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 53.1 52.5 55.5
Depreciation and amortization                 1.5 1.4 1.8
Operating income (loss)                 9.1 9.9 6.2
Total assets 381.9       476.6       381.9 476.6 414.2
Construction and acquisition expenditures                 7.3 0.1 26.9
Other Utility [Member] | WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 18.2 4.2 6.5
Depreciation and amortization                 0 0 0
Operating income (loss)                 (2.8) (2.5) (9.4)
Equity (income) loss from unconsolidated investments, net                 0 0 0
Total assets 350.6       489.4       350.6 489.4 366.9
Investments in equity method subsidiaries 0       0       0 0 0
Construction and acquisition expenditures                 0 0 0.5
Utility Business [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 3,225.1 3,042.3 3,174.5
Depreciation and amortization                 363.3 329.8 319.2
Operating income (loss)                 508.1 485.1 488.8
Interest expense, net of AFUDC                 135.5 136.8 146.6
Equity (income) loss from unconsolidated investments, net                 (43.7) (42.1) (38.7)
Interest income and other                 (0.4) (0.3) (0.2)
Income tax expense (benefit)                 49.3 74.8 78.3
Income from continuing operations, net of tax                 367.4 315.9 302.8
Loss from discontinued operations, net of tax                 0 0 0
Net income                 367.4 315.9 302.8
Preferred dividends                 17.9 15.9 18.3
Net income attributable to Alliant Energy common shareowners                 349.5 300.0 284.5
Total assets 10,610.4       10,219.6       10,610.4 10,219.6 9,137.5
Investments in equity method subsidiaries 279.1       264.3       279.1 264.3 246.5
Construction and acquisition expenditures                 731.6 1,025.5 608.1
Utility Business [Member] | IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 1,818.8 1,650.3 1,740.1
Depreciation and amortization                 191.1 188.9 179.1
Operating income (loss)                 212.0 200.3 208.4
Interest expense, net of AFUDC                 60.3 70.1 72.9
Interest income and other                 (0.3) (0.2) (0.2)
Income tax expense (benefit)                 (37.9) (19.8) (3.6)
Net income                 189.9 150.2 139.3
Preferred dividends                 16.3 12.6 15.0
Net income attributable to Alliant Energy common shareowners                 173.6 137.6 124.3
Total assets 5,806.0       5,457.0       5,806.0 5,457.0 5,093.5
Construction and acquisition expenditures                 400.2 307.5 293.7
Utility Business [Member] | WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 1,406.3 1,392.0 1,434.4
Depreciation and amortization                 172.2 140.9 140.1
Operating income (loss)                 296.1 284.8 280.4
Interest expense, net of AFUDC                 75.2 66.7 73.7
Equity (income) loss from unconsolidated investments, net                 (43.7) (42.1) (38.7)
Interest income and other                 (0.1) (0.1)  
Income tax expense (benefit)                 87.2 94.6 81.9
Net income                 177.5 165.7 163.5
Preferred dividends                 1.6 3.3 3.3
Net income attributable to Alliant Energy common shareowners                 175.9 162.4 160.2
Total assets 4,804.4       4,762.6       4,804.4 4,762.6 4,044.0
Investments in equity method subsidiaries 279.1       264.3       279.1 264.3 246.5
Construction and acquisition expenditures                 331.4 718.0 314.4
Non-Regulated [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 51.7 52.2 46.9
Depreciation and amortization                 7.6 2.6 1.8
Operating income (loss)                 25.8 34.6 24.5
Interest expense, net of AFUDC                 6.5 (2.0) (0.3)
Equity (income) loss from unconsolidated investments, net                 0 0.8 (0.6)
Interest income and other                 0 (3.7) (4.1)
Income tax expense (benefit)                 4.6 14.6 (9.1)
Income from continuing operations, net of tax                 14.7 24.9 38.6
Loss from discontinued operations, net of tax                 (5.9) (5.1) (19.5)
Net income                 8.8 19.8 19.1
Preferred dividends                 0 0 0
Net income attributable to Alliant Energy common shareowners                 8.8 19.8 19.1
Total assets 502.0       565.9       502.0 565.9 550.4
Investments in equity method subsidiaries 2.3       2.3       2.3 2.3 3.1
Construction and acquisition expenditures                 $ 66.7 $ 132.6 $ 65.3