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Regulatory Matters (Regulatory Assets) (Details) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,963.0 | $ 1,915.1
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,080.2 | 1,055.6
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 568.5 | 578.7
Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 108.6 | 105.9
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 46.4 | 30.7
WPL's EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 39.7 | 41.4
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.0 | 26.2
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 93.6 | 76.6
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,500.8 | 1,458.8
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,044.9 | 1,022.4
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 289.1 | 294.0
IPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 67.1 | 64.3
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 17.2 | 10.0
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.0 | 26.2
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 56.5 | 41.9
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 462.2 | 456.3
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 35.3 | 33.2
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 279.4 | 284.7
WPL [Member] | Asset retirement obligations [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 41.5 | 41.6
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 29.2 | 20.7
WPL [Member] | WPL's EGUs retired early [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 39.7 | 41.4
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 37.1 | $ 34.7

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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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