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Regulatory Matters (Electric Generating Units Retired Early (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,737.3 | $ 1,666.7
EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 111.6 | 63.8
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,279.0 | 1,231.6
IPL [Member] | EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 55.4 | 31.6
IPL [Member] | EGUs retired early [Member] | Sutherland Units 1 And 3 [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 27.6 |
Recovery period, authorized | 10 years |
IPL [Member] | EGUs retired early [Member] | ML Kapp [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 27.8 |
Recovery period, authorized | 10 years |
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 458.3 | 435.1
WPL [Member] | EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 56.2 | $ 32.2
WPL [Member] | EGUs retired early [Member] | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 27.4 |
Recovery period, authorized | 10 years |
WPL [Member] | EGUs retired early [Member] | Edgewater Unit 4 [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 28.8 |
Recovery period, authorized | 10 years |

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- Definition Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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