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Regulatory Matters (Assets Retired Early) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 2,010 | $ 1,845
Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 113 | 134
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,483 | 1,400
IPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 77 | 88
IPL [Member] | Assets retired early [Member] | Sutherland Units 1 and 3 [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 23 |
IPL [Member] | Assets retired early [Member] | M.L. Kapp Unit 2 [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 21 |
IPL [Member] | Assets retired early [Member] | Analog Electric Meters [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 33 |
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 527 | 445
WPL [Member] | Assets retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 36 | $ 46
WPL [Member] | Assets retired early [Member] | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 13 |
WPL [Member] | Assets retired early [Member] | Edgewater Unit 4 [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 23 |

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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://fasb.org/us-gaap/role/ref/legacyRef -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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