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Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2014 | Dec. 31, 2013
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Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,417.3 | $ 1,413.2
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 834.0 | 829.7
Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 351.8 | 355.3
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 68.5 | 65.7
Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 27.8 | 25.0
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 29.2 | 30.0
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 9.1 | 21.1
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 96.9 | 86.4
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,118.0 | 1,113.5
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 808.3 | 798.6
IPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 172.6 | 174.2
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 38.6 | 36.7
IPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 23.0 | 21.0
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 29.2 | 30.0
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 2.5 | 5.9
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 43.8 | 47.1
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 299.3 | 299.7
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 25.7 | 31.1
WPL [Member] | Pension and OPEB costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 179.2 | 181.1
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 29.9 | 29.0
WPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 4.8 | 4.0
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 6.6 | 15.2
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 53.1 | $ 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