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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets | $ 1,963.0 |  |  | $ 1,915.1
Regulatory liabilities | 696.6 |  |  | 681.0
IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets | 1,500.8 |  |  | 1,458.8
Regulatory liabilities | 448.3 |  |  | 430.8
WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets | 462.2 |  |  | 456.3
Regulatory liabilities | 248.3 |  |  | 250.2
2017 and 2018 Test Period Retail Electric [Member] | Alliant Energy and WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase in electric revenues | 22.0 |  |  |
2017 and 2018 Test Period Retail Gas [Member] | Alliant Energy and WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Increase in gas revenues | $ 2.0 |  |  |
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] | 2017 Test Year Retail Electric Fuel Related [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Under-collection of retail fuel-related costs |  |  | $ 8.0 |
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 subject to final true-up | $ 51.0 |  |  |
FERC [Member] | MISO Transmission Owner Complaints [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Refund received subject to final true-up | 40.0 |  |  |
FERC [Member] | MISO Transmission Owner Complaints [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Refund received subject to final true-up | 11.0 |  |  |
Commodity cost recovery [Member] | 2017 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory assets | 1.0 |  |  |
Commodity cost recovery [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities | 32.9 |  |  | 30.8
Commodity cost recovery [Member] | IPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities | 20.1 |  |  | 17.8
Commodity cost recovery [Member] | WPL [Member] |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory liabilities | 12.8 |  |  | $ 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 |  |  |

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