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Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value |  | $ 381.4 | $ 326.0 | $ 302.9
Equity (income) / loss |  | (44.8) | (39.6) | (33.8)
WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity (income) / loss |  | (0.7) | (39.8) | (35.1)
ATC Investment [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value | [1] | 274.2 | 317.6 |
Equity (income) / loss | [1] | $ (42.4) | (39.1) | (34.2)
Wind Investment in Oklahoma [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest |  | 50.00% |  |
Carrying value |  | $ 98.3 | 0.0 |
Equity (income) / loss |  | (1.8) | 0.0 | 0.0
Other [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value |  | 8.9 | 8.4 |
Equity (income) / loss |  | $ (0.6) | (0.5) | 0.4
ATC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest |  | 16.00% |  |
ATC [Member] | WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest |  | 0.00% |  |
Carrying value |  | $ 0.0 | 0.0 |
Equity (income) / loss |  | $ 0.0 | (39.1) | (34.2)
Wisconsin River Power Company [Member] | WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest |  | 50.00% |  |
Carrying value |  | $ 8.3 | 7.7 |
Equity (income) / loss |  | (0.7) | (0.7) | (0.9)
Totals [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value |  | 381.4 | 326.0 |
Equity (income) / loss |  | (44.8) | (39.6) | (33.8)
Totals [Member] | WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value |  | 8.3 | 7.7 |
Equity (income) / loss |  | $ (0.7) | $ (39.8) | $ (35.1)
ATC Holdco LLC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest |  | 20.00% |  |
[1] | As of December 31, 2017, Alliant Energy’s ATC Investment is comprised of a 16% ownership interest in ATC and a 20% ownership interest in ATC Holdco LLC, which are described below. In 2017, Alliant Energy’s investment in ATC decreased due to the impacts of Tax Reform. Refer to Note 11 for further discussion. Alliant Energy currently has the ability to exercise significant influence over ATC’s financial and operating policies through its participation on ATC’s Board of Directors. Refer to Note 18 for information regarding related party transactions with ATC.

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