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Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 1,413.2 | $ 1,612.4
Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 829.7 | 770.7
Pension and other postretirement benefits costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 355.3 | 549.2
AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 65.7 | 62.4
Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 30.0 | 30.0
Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 25.0 | 34.9
Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 21.1 | 40.2
Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 17.9 | 19.8
IPL's electric transmission cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 8.3 | 16.6
Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 60.2 | 88.6
IPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 1,113.5 | 1,217.9
IPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 798.6 | 746.2
IPL [Member] | Pension and other postretirement benefits costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 174.2 | 279.3
IPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 36.7 | 38.6
IPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 30.0 | 30.0
IPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 21.0 | 30.3
IPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 5.9 | 16.3
IPL [Member] | Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 12.2 | 13.6
IPL [Member] | IPL's electric transmission cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 8.3 | 16.6
IPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 26.6 | 47.0
WPL [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 299.7 | 394.5
WPL [Member] | Tax-related [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 31.1 | 24.5
WPL [Member] | Pension and other postretirement benefits costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 181.1 | 269.9
WPL [Member] | AROs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 29.0 | 23.8
WPL [Member] | Emission allowances [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 0 | 0
WPL [Member] | Environmental-related costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 4.0 | 4.6
WPL [Member] | Derivatives [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 15.2 | 23.9
WPL [Member] | Debt redemption costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 5.7 | 6.2
WPL [Member] | IPL's electric transmission cost recovery [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | 0 | 0
WPL [Member] | Other [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory assets | $ 33.6 | $ 41.6

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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