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Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified | Jun. 30, 2014 | Dec. 31, 2013
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Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,453.2 | $ 1,413.2
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 870.7 | 829.7
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 348.4 | 355.3
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 71.6 | 65.7
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 28.7 | 30.0
Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 27.2 | 25.0
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 12.5 | 21.1
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 94.1 | 86.4
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,152.6 | 1,113.5
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 844.1 | 798.6
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 171.2 | 174.2
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 40.9 | 36.7
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 28.7 | 30.0
IPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 22.5 | 21.0
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 6.1 | 5.9
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 39.1 | 47.1
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 300.6 | 299.7
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.6 | 31.1
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 177.2 | 181.1
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 30.7 | 29.0
WPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 4.7 | 4.0
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 6.4 | 15.2
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 55.0 | $ 39.3

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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
Name: | us-gaap_RegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant