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Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Amounts recognized on the balance sheets consist of: |  |
Pension and other benefit obligations | $ (504.0) | $ (489.9)
Pension Plans, Defined Benefit [Member] |  |
Amounts recognized on the balance sheets consist of: |  |
Non-current assets | 0.0 | 0.0
Current liabilities | (2.2) | (6.5)
Pension and other benefit obligations | (350.2) | (342.1)
Net amounts recognized at December 31 | (352.4) | (348.6)
Amounts recognized in Regulatory Assets and AOCL consist of: |  |
Net actuarial loss | 509.1 | 535.1
Prior service credit | (6.5) | (6.9)
Net amount recognized at December 31 | 502.6 | 528.2
Other Postretirement Benefits Plans [Member] |  |
Amounts recognized on the balance sheets consist of: |  |
Non-current assets | 8.8 | 3.2
Current liabilities | (9.1) | (8.6)
Pension and other benefit obligations | (110.9) | (108.9)
Net amounts recognized at December 31 | (111.2) | (114.3)
Amounts recognized in Regulatory Assets and AOCL consist of: |  |
Net actuarial loss | 47.4 | 52.6
Prior service credit | (1.3) | (1.5)
Net amount recognized at December 31 | 46.1 | 51.1
IPL [Member] |  |
Amounts recognized on the balance sheets consist of: |  |
Pension and other benefit obligations | (173.8) | (173.2)
IPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Amounts recognized on the balance sheets consist of: |  |
Non-current assets | 0.0 | 0.0
Current liabilities | (0.5) | (0.7)
Pension and other benefit obligations | (148.7) | (147.7)
Net amounts recognized at December 31 | (149.2) | (148.4)
Amounts recognized in Regulatory Assets and AOCL consist of: |  |
Net actuarial loss | 218.9 | 233.6
Prior service credit | (2.1) | (2.3)
Net amount recognized at December 31 | 216.8 | 231.3
IPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Amounts recognized on the balance sheets consist of: |  |
Non-current assets | 5.9 | 0.4
Current liabilities | (2.0) | (1.9)
Pension and other benefit obligations | (20.4) | (20.4)
Net amounts recognized at December 31 | (16.5) | (21.9)
Amounts recognized in Regulatory Assets and AOCL consist of: |  |
Net actuarial loss | 18.7 | 25.4
Prior service credit | 0.0 | 0.0
Net amount recognized at December 31 | 18.7 | 25.4
WPL [Member] |  |
Amounts recognized on the balance sheets consist of: |  |
Pension and other benefit obligations | (213.7) | (207.8)
WPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Amounts recognized on the balance sheets consist of: |  |
Non-current assets | 0.0 | 0.0
Current liabilities | (0.1) | (0.1)
Pension and other benefit obligations | (144.7) | (139.4)
Net amounts recognized at December 31 | (144.8) | (139.5)
Amounts recognized in Regulatory Assets and AOCL consist of: |  |
Net actuarial loss | 224.7 | 236.1
Prior service credit | (1.5) | (1.4)
Net amount recognized at December 31 | 223.2 | 234.7
WPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Amounts recognized on the balance sheets consist of: |  |
Non-current assets | 2.9 | 2.7
Current liabilities | (6.8) | (6.4)
Pension and other benefit obligations | (67.8) | (66.6)
Net amounts recognized at December 31 | (71.7) | (70.3)
Amounts recognized in Regulatory Assets and AOCL consist of: |  |
Net actuarial loss | 23.6 | 21.5
Prior service credit | (1.3) | (1.5)
Net amount recognized at December 31 | $ 22.3 | $ 20.0

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