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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2017 | Jun. 30, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets | $ 2,016.0 | $ 2,016.0 |  |  | $ 1,915.1
Regulatory liabilities | 665.8 | 665.8 |  |  | 681.0
IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets | 1,556.6 | 1,556.6 |  |  | 1,458.8
Regulatory liabilities | 415.7 | 415.7 |  |  | 430.8
WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets | 459.4 | 459.4 |  |  | 456.3
Regulatory liabilities | 250.1 | 250.1 |  |  | 250.2
2016 Test Year Retail Electric [Member] | Alliant Energy and IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Increase in electric margins | 20.0 | 20.0 |  |  |
2017 and 2018 Test Period Retail Electric [Member] | Alliant Energy and WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Increase in electric margins | 16.0 | 38.0 |  |  |
2017 and 2018 Test Period Retail Gas [Member] | Alliant Energy and WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Increase in gas margins | 2.0 | $ 4.0 |  |  |
2017 Test Year Retail Electric Fuel-related [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Annual bandwidth for fuel-related costs |  | 2.00% |  |  |
2016 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% |
Scenario, Forecast [Member] | 2017 and 2018 Test Period Retail Electric [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Authorized increase (decrease) in final rates, amount |  |  |  | $ 9.0 |
Authorized increase (decrease) in final rates, percent |  |  |  | 1.00% |
Increase in base rates |  |  |  | $ 60.0 |
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% |  |
Scenario, Forecast [Member] | 2017 Test Year Retail Electric Fuel-related [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Under-collection of retail fuel-related costs |  |  |  | $ 12.0 |
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 |  |  |
Other [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets | 96.3 | 96.3 |  |  | 76.6
Other [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets | 52.3 | 52.3 |  |  | 41.9
Other [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets | 44.0 | 44.0 |  |  | 34.7
Other [Member] | 2017 Test Year Retail Electric Fuel-related [Member] | Alliant Energy and WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory assets | 8.0 | 8.0 |  |  |
Commodity cost recovery [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities | 27.5 | 27.5 |  |  | 30.8
Commodity cost recovery [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities | 15.9 | 15.9 |  |  | 17.8
Commodity cost recovery [Member] | WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities | 11.6 | 11.6 |  |  | $ 13.0
Commodity cost recovery [Member] | 2016 Test Year Retail Electric Fuel-related [Member] | Alliant Energy and WPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Regulatory liabilities | $ 9.0 | $ 9.0 |  |  |
Sutherland Units 1 And 3 [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |
Requested recovery period |  |  |  | 10 years |

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