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Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2013 | Dec. 31, 2012
Commercial paper: |  |  |
Amount outstanding | $ 243.4 |  | $ 217.5
Weighted average remaining maturity (in days) | 3 days |  |
Weighted average interest rates | 0.20% |  |
Available credit facility capacity | 721.6 | [1] |
Parent Company [Member] |  |  |
Commercial paper: |  |  |
Amount outstanding | 83.4 |  |
Weighted average remaining maturity (in days) | 2 days |  |
Weighted average interest rates | 0.30% |  |
Available credit facility capacity | 216.6 | [1] |
IPL [Member] |  |  |
Commercial paper: |  |  |
Amount outstanding | 0 |  | 26.3
Available credit facility capacity | 265.0 | [1] |
WPL [Member] |  |  |
Commercial paper: |  |  |
Amount outstanding | 160.0 |  | 86.6
Weighted average remaining maturity (in days) | 3 days |  |
Weighted average interest rates | 0.20% |  |
Available credit facility capacity | $ 240.0 | [1] |
[1] | Alliant Energy’s and IPL’s available credit facility capacities reflect outstanding commercial paper classified as both short- and long-term debt at March 31, 2013. Refer to Note 8(b) for further discussion of $35 million of commercial paper outstanding at March 31, 2013 classified as long-term debt on Alliant Energy’s and IPL’s Condensed Consolidated Balance Sheets.

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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 02 -Paragraph 19 -Subparagraph a(3) -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 2 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 4: 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 5: 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 + Details
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