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Redeemable Preferred Stock (Narrative) (Details) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 USD ($) Q board_member $ / shares | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) shares
Temporary Equity [Line Items] |  |  |
Reduction in commercial paper | $ 18.5 | $ (138.1) | $ 11.9
Preferred stock issuance costs |  |  | 5.4
IPL [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Cumulative preferred stock rate | 5.10% |  |
Reduction in commercial paper | $ 0.0 | 0.0 | (76.3)
Preferred stock issuance costs |  |  | $ 5.4
Number of board members to be elected (in members) | board_member | 2 |  |
Number of quarterly dividend requirements (in quarters) | Q | 6 |  |
IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock issued during period (in shares) | shares |  |  | 8,000,000
Cumulative preferred stock rate | 5.10% |  | 5.10%
Preferred stock issued during period, value |  |  | $ 200.0
Preferred stock redemption price per share (in dollars per share) | $ / shares | $ 25 |  |
IPL [Member] | Redeemable Preferred Stock [Member] | 8.375% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Cumulative preferred stock rate |  |  | 8.375%
Preferred stock redeemed during period (in shares) | shares |  |  | 6,000,000
Preferred stock redeemed during period, value |  |  | $ 150.0
Alliant Energy and IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock issuance costs |  |  | 5.0
Alliant Energy and IPL [Member] | Redeemable Preferred Stock [Member] | 8.375% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock redemption charge |  |  | 5.0
WPL [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Reduction in commercial paper | $ 19.9 | $ (183.7) | $ 97.1
WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% through 6.50% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Preferred stock redeemed during period (in shares) | shares |  |  | 1,049,225
Preferred stock redeemed during period, value |  |  | $ 61.0
WPL [Member] | Redeemable Preferred Stock [Member] | 4.40% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Cumulative preferred stock rate |  |  | 4.40%
WPL [Member] | Redeemable Preferred Stock [Member] | 6.50% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Cumulative preferred stock rate |  |  | 6.50%
Long-term debt [Member] | IPL [Member] | Redeemable Preferred Stock [Member] | 5.1% [Member] |  |  |
Temporary Equity [Line Items] |  |  |
Reduction in commercial paper |  |  | $ 40.0

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