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Benefit Plans (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 29, 2018
Defined Benefit Plan Disclosure [Line Items] |  |
Unrecognized compensation cost | $ 14.6 |
Alliant Energy common stock closing price on March 29, 2018 (in dollars per share) |  | $ 40.86
Minimum [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Unrecognized compensation cost recognized over a weighted average period | 1 year |
Maximum [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Unrecognized compensation cost recognized over a weighted average period | 2 years |
IPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Unrecognized compensation cost | $ 8.1 |
WPL [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Unrecognized compensation cost | $ 5.9 |

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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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