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Income Taxes (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2021 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Tax [Line Items] |  |  |  |  |
Statutory federal income tax rate |  |  | 21.00% | 35.00% | 35.00%
Income tax expense (benefits) Federal Tax Reform |  |  | $ (5.6) | $ (18.1) |
Overall income tax rate |  |  | 8.40% | 12.50% | 13.40%
Decrease in regulatory assets |  |  | $ (16.2) | $ (130.8) | $ (3.6)
Decrease in regulatory liabilities |  |  | $ 1.3 | $ (83.8) | $ (63.0)
IPL [Member] |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Statutory federal income tax rate |  |  | 21.00% | 35.00% | 35.00%
Income tax expense (benefits) Federal Tax Reform |  |  | $ (1.1) | $ 3.8 |
Overall income tax rate |  |  | (1.20%) | (5.00%) | (2.70%)
Decrease in regulatory assets |  |  | $ (20.2) | $ (126.2) | $ (54.7)
Decrease in regulatory liabilities |  |  | $ 0.6 | $ (71.2) | $ (67.3)
WPL [Member] |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Statutory federal income tax rate |  |  | 21.00% | 35.00% | 35.00%
Income tax expense (benefits) Federal Tax Reform |  |  | $ (5.5) | $ (14.5) |
Overall income tax rate |  |  | 14.80% | 24.90% | 32.60%
Decrease in regulatory assets |  |  | $ 4.0 | $ (4.7) | $ 51.1
Iowa Tax Reform [Member] | Alliant Energy and IPL [Member] |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Decrease in regulatory assets | $ 33.7 |  |  |  |
Decrease in regulatory liabilities | $ 7.3 |  |  |  |
State [Member] | IPL [Member] |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Overall income tax rate |  |  | 12.00% |  |
State [Member] | IPL [Member] | Scenario, Forecast [Member] |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Overall income tax rate |  | 9.80% |  |  |

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- Definition Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 + Details
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