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Regulatory Matters (Regulatory Assets) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory assets	$ 1,524.3	$ 1,413.2
Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	937.6	829.7
Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	345.4	355.3
AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	74.6	65.7
Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.0	30.0
Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	26.6	25.0
Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	23.3	21.1
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	88.8	86.4
IPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,226.8	1,113.5
IPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	910.6	798.6
IPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	170.2	174.2
IPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	43.1	36.7
IPL [Member] | Emission allowances [Member]
Regulatory Assets [Line Items]
Regulatory assets	28.0	30.0
IPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	22.0	21.0
IPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	17.7	5.9
IPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	35.2	47.1
WPL [Member]
Regulatory Assets [Line Items]
Regulatory assets	297.5	299.7
WPL [Member] | Tax-related [Member]
Regulatory Assets [Line Items]
Regulatory assets	27.0	31.1
WPL [Member] | Pension and OPEB costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	175.2	181.1
WPL [Member] | AROs [Member]
Regulatory Assets [Line Items]
Regulatory assets	31.5	29.0
WPL [Member] | Environmental-related costs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4.6	4.0
WPL [Member] | Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.6	15.2
WPL [Member] | Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 53.6	$ 39.3

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- Details

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Namespace Prefix:	us-gaap_
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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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