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Redeemable Preferred Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended | 1 Months Ended | 3 Months Ended | 12 Months Ended | 1 Months Ended | 3 Months Ended | 1 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012 | Mar. 31, 2013 IPL [Member] | Mar. 31, 2012 IPL [Member] | Dec. 31, 2012 IPL [Member] | Mar. 31, 2013 IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member] | Mar. 31, 2013 IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member] | Mar. 31, 2013 Alliant Energy and IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member] | Mar. 31, 2013 Alliant Energy and IPL [Member] Redeemable Preferred Stock [Member] 8.375% [Member] | Mar. 31, 2013 WPL [Member] | Mar. 31, 2012 WPL [Member] | Dec. 31, 2012 WPL [Member] | Mar. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 4.40% [Member] | Mar. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 6.50% [Member] | Mar. 31, 2013 WPL [Member] Redeemable Preferred Stock [Member] 4.40% through 6.50% [Member] | Mar. 31, 2013 Alliant Energy and WPL [Member] Redeemable Preferred Stock [Member] 4.40% through 6.50% [Member] | Mar. 31, 2013 Long-term Debt [Member] IPL [Member] Redeemable Preferred Stock [Member] 5.1% [Member]
Temporary Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock issued during period (in shares) |  |  |  |  |  | 8,000,000 |  |  |  |  |  |  |  |  |  |  |
Preferred stock series |  |  | 5.10% |  | 8.375% | 5.10% | 8.375% |  |  |  |  | 4.50% | 4.40% | 6.50% |  |  |
Preferred stock issued during period, value |  |  |  |  |  | $ 200 |  |  |  |  |  |  |  |  |  |  |
Reduction in commercial paper | 10.9 | (20.8) | (41.3) | 17.9 |  |  |  |  |  | 73.4 | (2.4) |  |  |  |  |  | 40.0
Preferred stock issuance costs |  |  |  |  |  |  |  | 5 |  |  |  |  |  |  |  |  |
Preferred stock redemption terms |  |  |  |  |  | On or after March 15, 2018 |  |  |  |  |  |  |  |  |  |  |
Preferred stock redemption price (in dollars per share) |  |  |  |  |  | $ 25 |  |  |  |  |  |  |  |  |  |  |
Preferred stock redeemed during period (in shares) |  |  |  |  |  |  | 6,000,000 |  |  |  |  |  |  |  | 1,049,225 |  |
Stock redeemed during period, value |  |  |  |  |  |  | 150 |  |  |  |  |  |  |  | 61 |  |
Preferred stock issuance charge | $ 10.2 | $ 4.0 | $ 8.6 | $ 3.2 |  |  |  |  | $ 5.0 | $ 1.6 | $ 0.8 |  |  |  |  | $ 1.0 |

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- Definition Direct costs (e.g., legal and accounting fees) associated with issuing stock that is deducted from additional paid in capital. Also includes any direct costs associated with stock issues under a shelf registration. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=6959260&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Technical Practice Aid (TPA) -Number 4110 -Paragraph 1, 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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