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Debt (Credit Facilities) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 | Dec. 31, 2012
Commercial paper: |  |  |  |
Amount outstanding | $ 279.4 |  | $ 217.5 |
Weighted average interest rates | 0.20% |  | 0.40% |
Weighted average remaining maturity | 4 days |  | 11 days |
Available credit facility capacity | 720.6 | [1] | 732.5 | [1]
IPL [Member] |  |  |  |
Commercial paper: |  |  |  |
Amount outstanding | 0 |  | 26.3 |
Weighted average interest rates |  |  | 0.40% |
Weighted average remaining maturity |  |  | 2 days |
Available credit facility capacity | 300.0 | [1] | 223.7 | [1]
WPL [Member] |  |  |  |
Commercial paper: |  |  |  |
Amount outstanding | 183.7 |  | 86.6 |
Weighted average interest rates | 0.10% |  | 0.30% |
Weighted average remaining maturity | 6 days |  | 19 days |
Available credit facility capacity | 216.3 | [1] | 313.4 | [1]
Long-term debt [Member] |  |  |  |
Commercial paper: |  |  |  |
Amount outstanding | 0 | [2] | 50.0 | [2]
Long-term debt [Member] | IPL [Member] |  |  |  |
Commercial paper: |  |  |  |
Amount outstanding | 0 | [2] | 50.0 | [2]
Long-term debt [Member] | Alliant Energy and IPL [Member] |  |  |  |
Commercial paper: |  |  |  |
Amount outstanding |  |  | $ 50.0 |
[1] | At December 31, 2012, Alliant Energy’s and IPL’s available credit facility capacities reflect outstanding commercial paper classified as both short- and long-term debt. Refer to Note 9(b) for further discussion of $50.0 million of commercial paper outstanding at December 31, 2012 classified as long-term debt on Alliant Energy’s and IPL’s Consolidated Balance Sheets. Alliant Energy and its subsidiaries did not have any commercial paper classified as long-term debt as of December 31, 2013.
[2] | At December 31, 2012, $50.0 million of commercial paper was recorded in “Long-term debt, net” on Alliant Energy’s and IPL’s Consolidated Balance Sheets due to the existence of long-term credit facilities that back-stop this commercial paper balance, along with Alliant Energy’s and IPL’s intent and ability to refinance these balances on a long-term basis.

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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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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + References Reference 1: 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(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition Weighted average interest rate of short-term debt outstanding. + References Reference 1: 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(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph b -Article 5 + Details
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