v2.4.0.6
Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Segment Reporting Information [Line Items]                      
Operating revenues $ 750.9 $ 887.6 $ 690.3 $ 765.7 $ 760.8 $ 870.9 $ 712.5 $ 877.2 $ 3,094.5 $ 3,221.4 $ 3,262.1
Depreciation and amortization                 332.4 321.0 287.3
Operating income (loss) 101.6 213.7 108.8 95.6 106.0 209.4 70.6 127.3 519.7 513.3 560.0
Interest expense, net of AFUDC                 134.8 146.3 144.8
Equity income from unconsolidated investments, net                 (41.3) (39.3) (38.1)
Interest income and other                 (4.0) (4.3) (4.6)
Income tax expense (benefit)                 89.4 69.2 147.7
Income from continuing operations, net of tax                 340.8 341.4 310.2
Loss from discontinued operations, net of tax                 (5.1) (19.5) (3.9)
Net income                 335.7 321.9 306.3
Preferred dividends                 15.9 18.3 18.7
Net income attributable to Alliant Energy common shareowners 68.3 150.7 65.9 34.9 57.0 122.0 51.1 73.5 319.8 303.6 287.6
Total assets 10,785.5       9,687.9       10,785.5 9,687.9 9,282.9
Investments in equity method subsidiaries 266.6       249.6       266.6 249.6 238.5
Construction and acquisition expenditures                 1,158.1 673.4 866.9
IPL and WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Number of reportable segments (in segments)                 3    
IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues 393.2 497.7 360.7 398.7 404.0 484.5 374.1 477.5 1,650.3 1,740.1 1,795.8
Depreciation and amortization                 188.9 179.1 177.0
Operating income (loss) 29.0 112.0 35.4 23.9 26.3 116.5 18.5 47.1 200.3 208.4 261.9
Interest income and other                 (0.2) (0.2) (0.5)
Income tax expense (benefit)                 (19.8) (3.6) 42.3
Net income 25.6 106.5 19.6 (1.5) 9.8 101.4 1.0 27.1 150.2 139.3 143.4
Preferred dividends                 12.6 15.0 15.4
Net income attributable to Alliant Energy common shareowners 22.4 103.3 16.6 (4.7) 6.6 98.3 (2.3) 21.7 137.6 124.3 128.0
Total assets 5,457.0       5,093.5       5,457.0 5,093.5  
WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues 345.2 376.6 315.7 354.5 344.5 374.6 326.8 388.5 1,392.0 1,434.4 1,423.6
Depreciation and amortization                 140.9 140.1 108.6
Operating income (loss) 65.3 93.4 61.5 64.6 73.2 86.9 45.5 74.8 284.8 280.4 278.8
Equity income from unconsolidated investments, net                 (42.1) (38.7) (37.8)
Interest income and other                 (0.1) 0 (0.1)
Income tax expense (benefit)                 94.6 81.9 98.3
Net income 41.0 56.7 36.1 31.9 42.7 51.4 25.0 44.4 165.7 163.5 152.3
Preferred dividends                 3.3 3.3 3.3
Net income attributable to Alliant Energy common shareowners 40.2 55.9 35.2 31.1 41.9 50.6 24.1 43.6 162.4 160.2 149.0
Total assets 4,762.6       4,044.0       4,762.6 4,044.0  
Electric [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 2,589.3 2,635.8 2,674.2
Depreciation and amortization                 299.3 289.0 255.1
Operating income (loss)                 426.2 444.2 489.8
Equity income from unconsolidated investments, net                 (42.1) (38.7) (37.8)
Total assets 8,438.8       7,524.5       8,438.8 7,524.5 7,227.2
Investments in equity method subsidiaries 264.3       246.5       264.3 246.5 236.0
Construction and acquisition expenditures                 994.0 542.7 729.1
Electric [Member] | IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 1,371.1 1,408.3 1,464.3
Depreciation and amortization                 171.2 161.3 157.1
Operating income (loss)                 166.2 181.6 239.7
Total assets 4,500.9       4,208.2       4,500.9 4,208.2 4,025.3
Construction and acquisition expenditures                 291.0 245.6 298.8
Electric [Member] | WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 1,218.2 1,227.5 1,209.9
Depreciation and amortization                 128.1 127.7 98.0
Operating income (loss)                 260.0 262.6 250.1
Equity income from unconsolidated investments, net                 (42.1) (38.7) (37.8)
Total assets 3,937.9       3,316.3       3,937.9 3,316.3 3,201.9
Investments in equity method subsidiaries 264.3       246.5       264.3 246.5 236.0
Construction and acquisition expenditures                 703.0 297.1 430.3
Gas [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 396.3 476.7 480.6
Depreciation and amortization                 29.1 28.4 25.2
Operating income (loss)                 51.5 47.8 53.4
Equity income from unconsolidated investments, net                 0 0 0
Total assets 814.8       831.9       814.8 831.9 817.6
Investments in equity method subsidiaries 0       0       0 0 0
Construction and acquisition expenditures                 31.4 38.0 39.9
Gas [Member] | IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 226.7 276.3 274.3
Depreciation and amortization                 16.3 16.0 14.6
Operating income (loss)                 24.2 20.6 23.6
Total assets 479.5       471.1       479.5 471.1 460.3
Construction and acquisition expenditures                 16.4 21.2 20.0
Gas [Member] | WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 169.6 200.4 206.3
Depreciation and amortization                 12.8 12.4 10.6
Operating income (loss)                 27.3 27.2 29.8
Equity income from unconsolidated investments, net                 0 0 0
Total assets 335.3       360.8       335.3 360.8 357.3
Investments in equity method subsidiaries 0       0       0 0 0
Construction and acquisition expenditures                 15.0 16.8 19.9
Other Utility [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 56.7 62.0 64.6
Depreciation and amortization                 1.4 1.8 5.3
Operating income (loss)                 7.4 (3.2) (2.5)
Equity income from unconsolidated investments, net                 0 0 0
Total assets 966.0       781.1       966.0 781.1 782.4
Investments in equity method subsidiaries 0       0       0 0 0
Construction and acquisition expenditures                 0.1 27.4 64.3
Other Utility [Member] | IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 52.5 55.5 57.2
Depreciation and amortization                 1.4 1.8 5.3
Operating income (loss)                 9.9 6.2 (1.4)
Total assets 476.6       414.2       476.6 414.2 452.0
Construction and acquisition expenditures                 0.1 26.9 64.0
Other Utility [Member] | WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 4.2 6.5 7.4
Depreciation and amortization                 0 0 0
Operating income (loss)                 (2.5) (9.4) (1.1)
Equity income from unconsolidated investments, net                 0 0 0
Total assets 489.4       366.9       489.4 366.9 330.4
Investments in equity method subsidiaries 0       0       0 0 0
Construction and acquisition expenditures                 0 0.5 0.3
Total [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 3,042.3 3,174.5 3,219.4
Depreciation and amortization                 329.8 319.2 285.6
Operating income (loss)                 485.1 488.8 540.7
Interest expense, net of AFUDC                 136.8 146.6 142.8
Equity income from unconsolidated investments, net                 (42.1) (38.7) (37.8)
Interest income and other                 0.3 0.2 (0.6)
Income tax expense (benefit)                 74.8 78.3 140.6
Income from continuing operations, net of tax                 315.9 302.8 295.7
Loss from discontinued operations, net of tax                 0 0 0
Net income                 315.9 302.8 295.7
Preferred dividends                 15.9 18.3 18.7
Net income attributable to Alliant Energy common shareowners                 300.0 284.5 277.0
Total assets 10,219.6       9,137.5       10,219.6 9,137.5 8,827.2
Investments in equity method subsidiaries 264.3       246.5       264.3 246.5 236.0
Construction and acquisition expenditures                 1,025.5 608.1 833.3
Total [Member] | IPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 1,650.3 1,740.1 1,795.8
Depreciation and amortization                 188.9 179.1 177.0
Operating income (loss)                 200.3 208.4 261.9
Interest expense, net of AFUDC                 70.1 72.9 76.7
Interest income and other                 (0.2) (0.2) (0.5)
Income tax expense (benefit)                 (19.8) (3.6) 42.3
Net income                 150.2 139.3 143.4
Preferred dividends                 12.6 15.0 15.4
Net income attributable to Alliant Energy common shareowners                 137.6 124.3 128.0
Total assets 5,457.0       5,093.5       5,457.0 5,093.5 4,937.6
Construction and acquisition expenditures                 307.5 293.7 382.8
Total [Member] | WPL [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 1,392.0 1,434.4 1,423.6
Depreciation and amortization                 140.9 140.1 108.6
Operating income (loss)                 284.8 280.4 278.8
Interest expense, net of AFUDC                 66.7 73.7 66.1
Equity income from unconsolidated investments, net                 (42.1) (38.7) (37.8)
Interest income and other                 (0.1)   (0.1)
Income tax expense (benefit)                 94.6 81.9 98.3
Net income                 165.7 163.5 152.3
Preferred dividends                 3.3 3.3 3.3
Net income attributable to Alliant Energy common shareowners                 162.4 160.2 149.0
Total assets 4,762.6       4,044.0       4,762.6 4,044.0 3,889.6
Investments in equity method subsidiaries 264.3       246.5       264.3 246.5 236.0
Construction and acquisition expenditures                 718.0 314.4 450.5
Non-Regulated [Member]
                      
Segment Reporting Information [Line Items]                      
Operating revenues                 52.2 46.9 42.7
Depreciation and amortization                 2.6 1.8 1.7
Operating income (loss)                 34.6 24.5 19.3
Interest expense, net of AFUDC                 (2.0) (0.3) 2.0
Equity income from unconsolidated investments, net                 0.8 (0.6) (0.3)
Interest income and other                 3.7 4.1 (4.0)
Income tax expense (benefit)                 14.6 (9.1) 7.1
Income from continuing operations, net of tax                 24.9 38.6 14.5
Loss from discontinued operations, net of tax                 (5.1) (19.5) (3.9)
Net income                 19.8 19.1 10.6
Preferred dividends                 0 0 0
Net income attributable to Alliant Energy common shareowners                 19.8 19.1 10.6
Total assets 565.9       550.4       565.9 550.4 455.7
Investments in equity method subsidiaries 2.3       3.1       2.3 3.1 2.5
Construction and acquisition expenditures                 $ 132.6 $ 65.3 $ 33.6