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Investments (Tables) | 6 Months Ended
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Jun. 30, 2013
Schedule of Investments [Line Items] |
Unconsolidated Equity Investments | Equity (income) loss from Alliant Energy’s and WPL’s unconsolidated investments accounted for under the equity method of accounting for the three and six months ended June 30 was as follows (in millions):
| Alliant Energy |  | WPL
| Three Months |  | Six Months |  | Three Months |  | Six Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
American Transmission Company LLC (ATC) |  | ($10.6 | ) |  |  | ($10.3 | ) |  |  | ($20.9 | ) |  |  | ($20.2 | ) |  |  | ($10.6 | ) |  |  | ($10.3 | ) |  |  | ($20.9 | ) |  |  | ($20.2 | )
Other | (0.3 | ) |  | (0.3 | ) |  | (0.7 | ) |  | 0.2 |  |  | (0.2 | ) |  | (0.3 | ) |  | (0.7 | ) |  | (0.5 | )
|  | ($10.9 | ) |  |  | ($10.6 | ) |  |  | ($21.6 | ) |  |  | ($20.0 | ) |  |  | ($10.8 | ) |  |  | ($10.6 | ) |  |  | ($21.6 | ) |  |  | ($20.7 | )
Summary Financial Information | Summary financial information from the unaudited financial statements of ATC for the three and six months ended June 30 was as follows (in millions):
| Three Months |  | Six Months
| 2013 |  | 2012 |  | 2013 |  | 2012
Operating revenues |  | $152.2 |  |  |  | $152.1 |  |  |  | $303.9 |  |  |  | $299.8 |
Operating income | 82.2 |  |  | 80.4 |  |  | 164.2 |  |  | 158.5 |
Net income | 61.3 |  |  | 59.3 |  |  | 121.8 |  |  | 117.4 |
WPL [Member] |
Schedule of Investments [Line Items] |
Unconsolidated Equity Investments | Equity (income) loss from Alliant Energy’s and WPL’s unconsolidated investments accounted for under the equity method of accounting for the three and six months ended June 30 was as follows (in millions):
| Alliant Energy |  | WPL
| Three Months |  | Six Months |  | Three Months |  | Six Months
| 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012 |  | 2013 |  | 2012
American Transmission Company LLC (ATC) |  | ($10.6 | ) |  |  | ($10.3 | ) |  |  | ($20.9 | ) |  |  | ($20.2 | ) |  |  | ($10.6 | ) |  |  | ($10.3 | ) |  |  | ($20.9 | ) |  |  | ($20.2 | )
Other | (0.3 | ) |  | (0.3 | ) |  | (0.7 | ) |  | 0.2 |  |  | (0.2 | ) |  | (0.3 | ) |  | (0.7 | ) |  | (0.5 | )
|  | ($10.9 | ) |  |  | ($10.6 | ) |  |  | ($21.6 | ) |  |  | ($20.0 | ) |  |  | ($10.8 | ) |  |  | ($10.6 | ) |  |  | ($21.6 | ) |  |  | ($20.7 | )
Summary Financial Information | Summary financial information from the unaudited financial statements of ATC for the three and six months ended June 30 was as follows (in millions):
| Three Months |  | Six Months
| 2013 |  | 2012 |  | 2013 |  | 2012
Operating revenues |  | $152.2 |  |  |  | $152.1 |  |  |  | $303.9 |  |  |  | $299.8 |
Operating income | 82.2 |  |  | 80.4 |  |  | 164.2 |  |  | 158.5 |
Net income | 61.3 |  |  | 59.3 |  |  | 121.8 |  |  | 117.4 |

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- Definition Schedule of Summary Financial Information [Table Text Block] + References No definition available. + Details
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- Definition Tabular disclosure of equity method investments in common stock. The disclosure may include: (a) the name of each investee or group of investments for which combined disclosure is appropriate, (2) the percentage ownership of common stock, (3) the difference, if any, between the carrying amount of an investment and the value of the underlying equity in the net assets and the accounting treatment of difference, if any, and (4) the aggregate value of each identified investment based on its quoted market price, if available. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 18 -Paragraph 20 -Subparagraph a, b -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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