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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2021 | Dec. 31, 2019 | Dec. 31, 2017
Regulatory Matters [Line Items] |  |  |  |  |  |  |
State income tax rate | 5.30% | 13.20% | 8.80% | 14.90% |  |  |
Decrease in regulatory assets |  |  | $ (27.4) | $ 108.9 |  |  |
Increase in regulatory liabilities |  |  | 14.7 | $ (64.8) |  |  |
Regulatory assets | $ 1,694.7 |  | 1,694.7 |  |  |  | $ 1,666.7
Refunded Federal Tax Reform benefits | 20.0 |  | 47.0 |  |  |  |
Regulatory liabilities | $ 1,505.6 |  | $ 1,505.6 |  |  |  | 1,497.2
IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
State income tax rate | (4.80%) | (13.20%) | (0.20%) | (9.80%) |  |  |
Decrease in regulatory assets |  |  | $ (14.0) | $ 107.8 |  |  |
Increase in regulatory liabilities |  |  | 7.5 | $ (49.6) |  |  |
Regulatory assets | $ 1,249.9 |  | 1,249.9 |  |  |  | 1,231.6
Refunded Federal Tax Reform benefits | 9.0 |  | 16.0 |  |  |  |
Regulatory liabilities | $ 762.6 |  | $ 762.6 |  |  |  | 755.4
WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
State income tax rate | 11.80% | 31.60% | 14.20% | 31.10% |  |  |
Decrease in regulatory assets |  |  | $ (13.4) | $ 1.1 |  |  |
Regulatory assets | $ 444.8 |  | 444.8 |  |  |  | 435.1
Refunded Federal Tax Reform benefits | 11.0 |  | 31.0 |  |  |  |
Regulatory liabilities | 743.0 |  | 743.0 |  |  |  | 741.8
Iowa tax reform [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Decrease in regulatory assets |  |  | 33.7 |  |  |  |
Increase in regulatory liabilities |  |  | 7.3 |  |  |  |
2017 Test Year Retail Gas [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Interim rate increase, amount | $ 11.0 |  |  |  |  |  |
Interim rate increase, percent | 5.00% |  |  |  |  |  |
2016 Test Year Retail Electric [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Interim rate increase, amount |  |  |  |  |  |  | $ 102.0
Interim rate increase, percent |  |  |  |  |  |  | 7.00%
Approved rate increase, amount |  |  | $ 130.0 |  |  |  |
Approved rate increase, percent |  |  | 9.00% |  |  |  |
Scenario, Forecast [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Recovery period, authorized |  |  |  |  |  | 10 years |
Scenario, Forecast [Member] | 2017 Test Year Retail Gas [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Requested rate increase, amount |  |  |  |  |  | $ 20.0 |
Requested rate increase, percent |  |  |  |  |  | 8.00% |
Requested rate increase, amended amount |  |  |  |  |  | $ 14.0 |
Requested rate increase, amended percent |  |  |  |  |  | 6.00% |
Revenue Subject to Refund [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | $ 22.0 |  | $ 22.0 |  |  |  |
Revenue Subject to Refund [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 19.0 |  | 19.0 |  |  |  |
Revenue Subject to Refund [Member] | WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 3.0 |  | 3.0 |  |  |  |
Revenue Subject to Refund [Member] | 2017 and 2018 Test Period Retail Electric and Gas [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory liabilities | 12.0 |  | $ 12.0 |  |  |  |
State [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
State income tax rate |  |  | 12.00% |  |  |  |
State [Member] | Scenario, Forecast [Member] | IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
State income tax rate |  |  |  |  | 9.80% |  |
Net book value of M.L. Kapp [Member] | Alliant Energy and IPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | 29.0 |  | $ 29.0 |  |  |  |
Net book value of Edgewater Unit 4 [Member] | Alliant Energy and WPL [Member] |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Regulatory assets | $ 24.0 |  | $ 24.0 |  |  |  |

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