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Investments (Unconsolidated Equity Investments) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Schedule of Investments [Line Items] |  |  |  |
Equity (income) loss from unconsolidated investments, net | $ (11.1) | $ (11.5) | $ (28.9) | $ (34.2)
WPL [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Equity (income) loss from unconsolidated investments, net | (11.1) | (11.4) | (30.2) | (34.2)
ATC [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Equity (income) loss from unconsolidated investments, net | (10.9) | (11.2) | (29.6) | (33.5)
ATC [Member] | WPL [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Equity (income) loss from unconsolidated investments, net | (10.9) | (11.2) | (29.6) | (33.5)
Other [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Equity (income) loss from unconsolidated investments, net | (0.2) | (0.3) | 0.7 | (0.7)
Other [Member] | WPL [Member] |  |  |  |
Schedule of Investments [Line Items] |  |  |  |
Equity (income) loss from unconsolidated investments, net | $ (0.2) | $ (0.2) | $ (0.6) | $ (0.7)

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