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Benefit Plans (Defined Contribution Retirement Plans) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
401(k) costs | $ 6.4 | $ 5.3 | $ 18.7 | $ 17.3
IPL [Member] |  |  |  |
401(k) costs | 3.3 | 2.7 | 9.6 | 8.4
WPL [Member] |  |  |  |
401(k) costs | $ 2.9 | $ 2.4 | $ 8.4 | $ 8.3

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- Definition The amount of the cost recognized during the period for defined contribution plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 + Details
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Period Type: | duration

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- Details
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Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |