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Benefit Plans (Defined Contribution Retirement Plans) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015
401(k) costs | $ 5.7 | $ 5.6 | $ 11.9 | $ 12.3
IPL [Member] |  |  |  |
401(k) costs | 2.9 | 2.9 | 6.0 | 6.3
WPL [Member] |  |  |  |
401(k) costs | $ 2.6 | $ 2.6 | $ 5.4 | $ 5.5

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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=49170846&loc=d3e28014-114942 + Details
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- Details
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- Details
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