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Income Taxes (Schedule Of Unrecognized Tax Benefits) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Uncertain Tax Positions [Roll Forward] |  |  |  |  |  |
Balance, January 1 | $ 23.5 | [1] | $ 66.7 | [1] | $ 101.7 |
Additions based on tax positions related to the current year | 0.7 |  | 0.7 |  | 3.8 |
Additions for tax positions of prior years (a) | 0 | [2] | 0 | [2] | 9.1 | [2]
Reductions for tax positions of prior years (b) | (23.5) | [3] | (43.9) | [3] | (31.8) | [3]
Settlements with taxing authorities | 0 |  | 0 |  | (16.1) |
Balance, December 31 | 0.7 | [1] | 23.5 | [1] | 66.7 | [1]
IPL [Member] |  |  |  |  |  |
Uncertain Tax Positions [Roll Forward] |  |  |  |  |  |
Balance, January 1 | 10.9 | [1] | 33.0 | [1] | 58.4 |
Additions based on tax positions related to the current year | 0 |  | 0.1 |  | 2.5 |
Additions for tax positions of prior years (a) | 0 | [2] | 0 | [2] | 3.8 | [2]
Reductions for tax positions of prior years (b) | (10.9) | [3] | (22.2) | [3] | (22.7) | [3]
Settlements with taxing authorities | 0 |  | 0 |  | (9.0) |
Balance, December 31 | 0 | [1] | 10.9 | [1] | 33.0 | [1]
WPL [Member] |  |  |  |  |  |
Uncertain Tax Positions [Roll Forward] |  |  |  |  |  |
Balance, January 1 | 12.6 | [1] | 33.7 | [1] | 42.1 |
Additions based on tax positions related to the current year | 0.7 |  | 0.6 |  | 1.3 |
Additions for tax positions of prior years (a) | 0 | [2] | 0 | [2] | 5.2 | [2]
Reductions for tax positions of prior years (b) | (12.6) | [3] | (21.7) | [3] | (8.0) | [3]
Settlements with taxing authorities | 0 |  | 0 |  | (6.9) |
Balance, December 31 | 0.7 | [1] | 12.6 | [1] | 33.7 | [1]
Regulatory Liabilities [Member] | Alliant Energy And IPL [Member] |  |  |  |  |  |
Uncertain Tax Positions [Roll Forward] |  |  |  |  |  |
Balance, December 31 | $ 0 | [1] | $ 10.0 | [1] |  |
[1] | At December 31, 2012 and 2011, $0 and $10 million, respectively, of uncertain tax positions balances for both Alliant Energy and IPL included amounts recorded in regulatory liability accounts.
[2] | The additions for tax positions of prior years were related to positions taken by Alliant Energy, IPL and WPL on their federal and state tax returns related to the capitalization and dispositions of property.
[3] | In 2012, the reductions for tax positions of prior years were due to the finalization of Alliant Energy’s federal income tax return audits for calendar years 2005 through 2009. In 2011, the reductions for tax positions of prior years were related to guidance published by the IRS clarifying the treatment of repair expenditures for electric distribution property. In 2010, the reductions of tax positions of prior years were primarily related to deductions taken by Alliant Energy, IPL and WPL on their federal and state tax returns that were settled under audit for amounts less than the reductions of tax positions recorded.

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- Definition The gross amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Interpretation (FIN) -Number 48 -Paragraph 21 -Subparagraph a -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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- Definition The gross amount of decreases in unrecognized tax benefits resulting from tax positions taken in prior period tax returns, excluding amounts pertaining to examined tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Interpretation (FIN) -Number 48 -Paragraph 21 -Subparagraph a(1) -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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- Definition The gross amount of decreases in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Interpretation (FIN) -Number 48 -Paragraph 21 -Subparagraph a(3) -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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- Definition The gross amount of increases in unrecognized tax benefits resulting from tax positions that have been or will be taken in the tax return for the current period, excluding amounts pertaining to examined tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Interpretation (FIN) -Number 48 -Paragraph 21 -Subparagraph a(2) -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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- Definition The gross amount of increases in unrecognized tax benefits resulting from tax positions taken in prior period tax returns, excluding amounts pertaining to examined tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Interpretation (FIN) -Number 48 -Paragraph 21 -Subparagraph a(1) -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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