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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Increase (decrease) in regulatory liabilities |  |  | $ 64.8 | $ 66.5 |  |  |
Depreciation and amortization | $ 120.7 | $ 104.1 | 342.7 | 308.7 |  |  |
Regulatory liabilities | 628.5 |  | 628.5 |  |  |  | $ 681.0
Regulatory assets | 2,036.5 |  | 2,036.5 |  |  |  | 1,915.1
IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Increase (decrease) in regulatory liabilities |  |  | 49.6 | 64.5 |  |  |
Other operation and maintenance | 104.4 | 94.8 | 288.7 | 279.8 |  |  |
Depreciation and amortization | 66.2 | 52.7 | 181.0 | 157.8 |  |  |
Regulatory liabilities | 384.8 |  | 384.8 |  |  |  | 430.8
Regulatory assets | 1,590.9 |  | 1,590.9 |  |  |  | 1,458.8
WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Other operation and maintenance | 66.1 | 54.2 | 179.7 | 157.2 |  |  |
Depreciation and amortization | 53.6 | $ 48.7 | 158.8 | $ 143.5 |  |  |
Regulatory liabilities | 243.7 |  | 243.7 |  |  |  | 250.2
Regulatory assets | 445.6 |  | 445.6 |  |  |  | 456.3
2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Write-down of regulatory assets | 9.0 |  |  |  |  |  |
2016 Test Year Retail Electric [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Increase in base rates | 34.0 |  | 54.0 |  |  |  |
2017 and 2018 Test Period Retail Electric [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Increase in base rates | 4.0 |  | 42.0 |  |  |  |
2017 and 2018 Test Period Retail Gas [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Increase in base rates | 2.0 |  | $ 6.0 |  |  |  |
2016 Test Year Retail Electric Fuel-related [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Annual bandwidth for fuel-related costs |  |  | 2.00% |  |  |  |
2017 Test Year Retail Electric Fuel-related [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Annual bandwidth for fuel-related costs |  |  | 2.00% |  |  |  |
Scenario, Forecast [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Requested rate increase (decrease), amount |  |  |  |  | $ 176.0 |  |
Requested rate increase (decrease), percent |  |  |  |  | 12.00% |  |
Interim rate increase (decrease), amount |  |  |  |  |  | $ 102.0 |
Interim rate increase (decrease), percent |  |  |  |  |  | 7.00% |
Settlement agreement rate increase, amount |  |  |  |  | $ 130.0 |  |
Settlement agreement rate increase, percent |  |  |  |  | 9.00% |  |
Scenario, Forecast [Member] | 2017 and 2018 Test Period Retail Electric [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Increase in base rates |  |  |  |  |  | $ 60.0 |
Authorized increase (decrease) in final rates, amount |  |  |  |  |  | $ 9.0 |
Authorized increase (decrease) in final rates, percent |  |  |  |  |  | 1.00% |
Reduction in fuel-related costs |  |  |  |  |  | $ 51.0 |
Scenario, Forecast [Member] | 2017 and 2018 Test Period Retail Gas [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  |  |  |  | $ 9.0 |
Authorized increase (decrease) in final rates, percent |  |  |  |  |  | 13.00% |
Scenario, Forecast [Member] | 2018 Test Year Retail Electric Fuel-related [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Requested rate increase (decrease), amount |  |  |  |  | $ 6.0 |  |
Requested rate increase (decrease), percent |  |  |  |  | 1.00% |  |
FERC [Member] | MISO Transmission Owner Complaints [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Base return on equity for refund period from November 12, 2013 through February 11, 2015, percent |  |  | 10.32% |  |  |  |
Refund received, after final true-up |  |  | $ 50.0 |  |  |  |
FERC [Member] | MISO Transmission Owner Complaints [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Refund received, after final true-up |  |  | 39.0 |  |  |  |
FERC [Member] | MISO Transmission Owner Complaints [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Refund received, after final true-up |  |  | 11.0 |  |  |  |
Asset retirement obligations [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 107.9 |  | 107.9 |  |  |  | 105.9
Asset retirement obligations [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 71.9 |  | 71.9 |  |  |  | 64.3
Asset retirement obligations [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 36.0 |  | 36.0 |  |  |  | 41.6
Asset retirement obligations [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Depreciation and amortization | 5.0 |  |  |  |  |  |
Asset retirement obligations [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Requested recovery period |  |  |  |  | 4 years |  |
Other [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 90.0 |  | 90.0 |  |  |  | 76.6
Other [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 51.0 |  | 51.0 |  |  |  | 41.9
Other [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 39.0 |  | 39.0 |  |  |  | 34.7
Other [Member] | 2017 Test Year Retail Electric Fuel-related [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 3.0 |  | 3.0 |  |  |  |
IPL's tax benefit riders [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 45.0 |  | 45.0 |  |  |  | 83.5
IPL's tax benefit riders [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 45.0 |  | 45.0 |  |  |  | 83.5
IPL's tax benefit riders [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Increase (decrease) in regulatory liabilities |  |  | 17.0 |  |  |  |
Commodity cost recovery [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 21.2 |  | 21.2 |  |  |  | 30.8
Commodity cost recovery [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 15.0 |  | 15.0 |  |  |  | 17.8
Commodity cost recovery [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 6.2 |  | 6.2 |  |  |  | $ 13.0
Commodity cost recovery [Member] | 2016 Test Year Retail Electric Fuel-related [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 3.0 |  | 3.0 |  |  |  |
Commodity cost recovery [Member] | 2017 Test Year Retail Electric Fuel-related [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 6.0 |  | $ 6.0 |  |  |  |
Sutherland Units 1 And 3 [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Other operation and maintenance | $ 4.0 |  |  |  |  |  |
Sutherland Units 1 And 3 [Member] | IUB [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Requested recovery period |  |  |  |  | 10 years |  |
Sutherland Units 1 And 3 [Member] | FERC [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Requested recovery period |  |  | 10 years |  |  |  |

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