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Income Taxes (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Tax [Line Items] |  |  |  |
Statutory federal income tax rate |  | 35.00% | 35.00% | 35.00%
Other assets, deferred charges and other |  | $ 41.0 |  |
Scenario, Forecast [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Deductions in income tax return | $ 670.0 |  |  |
IPL [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Statutory federal income tax rate |  | 35.00% | 35.00% | 35.00%
Other assets, deferred charges and other |  | $ 0.0 |  |
IPL [Member] | Scenario, Forecast [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Deductions in income tax return | 500.0 |  |  |
WPL [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Statutory federal income tax rate |  | 35.00% | 35.00% | 35.00%
Other assets, deferred charges and other |  | $ 0.0 |  |
WPL [Member] | Scenario, Forecast [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Deductions in income tax return | $ 140.0 |  |  |
Subsequent Event [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Statutory federal income tax rate | 21.00% |  |  |
Subsequent Event [Member] | IPL [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Statutory federal income tax rate | 21.00% |  |  |
Subsequent Event [Member] | WPL [Member] |  |  |  |
Income Tax [Line Items] |  |  |  |
Statutory federal income tax rate | 21.00% |  |  |

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- Definition Percentage of domestic federal statutory tax rate applicable to pretax income (loss). + References Reference 1: 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=65882285&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 3: 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=65882285&loc=d3e32698-109319 + Details
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