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Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended |  | 9 Months Ended |  |  | 1 Months Ended
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2013 | Sep. 30, 2012 | Dec. 31, 2012 | Sep. 30, 2013 Alliant Energy and IPL [Member] Long-term Debt [Member] | Sep. 30, 2013 IPL [Member] | Dec. 31, 2012 IPL [Member] | Oct. 31, 2013 4.7% senior debentures due 2043 Subsequent Event [Member] IPL [Member] Senior Debentures [Member]
Debt Instrument [Line Items] |  |  |  |  |  |  |
Commercial paper | $ 237.3 |  | $ 217.5 | $ 65.0 | $ 0 | $ 26.3 |
Remaining maturity (in days) |  |  |  | 2 days |  |  |
Weighted average interest rate |  |  |  | 0.25% |  |  | 4.70%
Proceeds from issuance of long-term debt | $ 0 | $ 75.0 |  |  |  |  | $ 250.0

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- Definition Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(2)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph a(3) -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 2 -Article 9 + Details
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