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Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value |  | $ 302.9 | $ 296.6 | $ 281.4
Equity (income) / loss |  | (33.8) | (40.4) | (43.7)
WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Equity (income) / loss |  | $ (35.1) | (42.8) | (43.7)
ATC [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest | [1] | 16.00% |  |
Carrying value | [1] | $ 293.3 | 286.5 |
Equity (income) / loss | [1] | $ (34.2) | (41.9) | (42.7)
ATC [Member] | WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest | [1] | 16.00% |  |
Carrying value | [1] | $ 293.3 | 286.5 |
Equity (income) / loss | [1] | $ (34.2) | (41.9) | (42.7)
Wisconsin River Power Company [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest |  | 50.00% |  |
Carrying value |  | $ 8.7 | 7.8 |
Equity (income) / loss |  | $ (0.9) | (0.9) | (1.0)
Wisconsin River Power Company [Member] | WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Ownership interest |  | 50.00% |  |
Carrying value |  | $ 8.7 | 7.8 |
Equity (income) / loss |  | (0.9) | (0.9) | (1.0)
Other [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value |  | 0.9 | 2.3 |
Equity (income) / loss |  | 1.3 | 2.4 | 0.0
Totals [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value |  | 302.9 | 296.6 |
Equity (income) / loss |  | (33.8) | (40.4) | (43.7)
Totals [Member] | WPL [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Carrying value |  | 302.0 | 294.3 |
Equity (income) / loss |  | $ (35.1) | $ (42.8) | $ (43.7)
[1] | Alliant Energy and WPL have the ability to exercise significant influence over ATC’s financial and operating policies through their participation on ATC’s Board of Directors. Refer to Note 18 for information regarding related party transactions with ATC.

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