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Redeemable Preferred Stock (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 3 Months Ended | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2013 IPL [Member] Q board_member | Dec. 31, 2012 IPL [Member] | Dec. 31, 2011 IPL [Member] | Dec. 31, 2013 IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member] | Dec. 31, 2013 IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member] | Dec. 31, 2012 IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member] | Dec. 31, 2013 WPL [Member] | Dec. 31, 2012 WPL [Member] | Dec. 31, 2011 WPL [Member] | Dec. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 4.40% [Member] | Dec. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 6.50% [Member] | Dec. 31, 2013 Alliant Energy and IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member] | Mar. 31, 2013 Long-term debt [Member] | Dec. 31, 2013 Long-term debt [Member] IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member] | Dec. 31, 2013 Minimum [Member] WPL [Member] | Dec. 31, 2013 Maximum [Member] WPL [Member]
Temporary Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock issued during period (in shares) |  |  |  |  |  |  | 8,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative preferred stock rate |  |  |  | 5.10% | 8.375% |  | 5.10% | 8.375% | 8.375% | [1] |  | 4.50% |  | 4.40% | 6.50% |  |  |  | 4.40% | [2] | 6.50% | [2]
Preferred stock issued during period, value |  |  |  |  |  |  | $ 200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reduction in commercial paper | 11.9 | 164.7 | 55.4 | (76.3) | 69.2 | 7.1 |  |  |  |  | 97.1 | 60.9 | (21.7) |  |  |  | 65.0 | 40.0 |  |  |  |
Preferred stock issuance costs | $ (5.4) |  |  | $ (5.4) |  |  |  |  |  |  |  |  |  |  |  | $ 5.0 |  |  |  |  |  |
Number of board members to be elected (in members) |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of quarterly dividend requirements (in quarters) |  |  |  | 6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | In 2013, IPL redeemed all 6,000,000 outstanding shares of its 8.375% cumulative preferred stock for $150 million plus accrued and unpaid dividends to the redemption date. Alliant Energy and IPL recorded a $5 million charge in 2013 related to this transaction in “Preferred dividend requirements” in their Consolidated Statements of Income.
[2] | In 2013, WPL redeemed all 1,049,225 outstanding shares of its 4.40% through 6.50% cumulative preferred stock for $61 million plus accrued and unpaid dividends to the redemption date. Alliant Energy and WPL recorded a $1 million charge in 2013 related to this transaction in “Preferred dividend requirements” in their Consolidated Statements of Income.

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