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Regulatory Matters (Regulatory Liabilities) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 836.4 | $ 821.5
Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 420.0 | 418.9
IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 239.8 | 265.4
Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 64.1 | 52.7
Derivatives [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 33.3 | 7.2
IPL's electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 21.2 | 14.6
IPL's electric transmission assets sale [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 18.8 | 21.6
Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 39.2 | 41.1
IPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 604.0 | 614.9
IPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 279.3 | 277.7
IPL [Member] | IPL's tax benefit riders [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 239.8 | 265.4
IPL [Member] | Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 14.9 | 9.3
IPL [Member] | Derivatives [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 8.8 | 3.6
IPL [Member] | IPL's electric transmission cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 21.2 | 14.6
IPL [Member] | IPL's electric transmission assets sale [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 18.8 | 21.6
IPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 21.2 | 22.7
WPL [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 232.4 | 206.6
WPL [Member] | Cost of removal obligations [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 140.7 | 141.2
WPL [Member] | Energy efficiency cost recovery [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 49.2 | 43.4
WPL [Member] | Derivatives [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | 24.5 | 3.6
WPL [Member] | Other [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory liabilities | $ 18.0 | $ 18.4

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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities 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 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
Name: | us-gaap_RegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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