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Summary Of Significant Accounting Policies (Tables) | 3 Months Ended
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Mar. 31, 2013
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012
Tax-related |  | $781.9 |  |  |  | $770.7 |  |  |  | $757.2 |  |  |  | $746.2 |  |  |  | $24.7 |  |  |  | $24.5 |
Pension and other postretirement benefits costs | 541.4 |  |  | 549.2 |  |  | 275.6 |  |  | 279.3 |  |  | 265.8 |  |  | 269.9 |
Asset retirement obligations (AROs) | 62.9 |  |  | 62.4 |  |  | 38.5 |  |  | 38.6 |  |  | 24.4 |  |  | 23.8 |
Environmental-related costs | 32.2 |  |  | 34.9 |  |  | 27.7 |  |  | 30.3 |  |  | 4.5 |  |  | 4.6 |
Emission allowances | 30.0 |  |  | 30.0 |  |  | 30.0 |  |  | 30.0 |  |  | — |  |  | — |
Derivatives | 17.0 |  |  | 40.2 |  |  | 6.3 |  |  | 16.3 |  |  | 10.7 |  |  | 23.9 |
Other | 117.2 |  |  | 125.0 |  |  | 72.6 |  |  | 77.2 |  |  | 44.6 |  |  | 47.8 |
|  | $1,582.6 |  |  |  | $1,612.4 |  |  |  | $1,207.9 |  |  |  | $1,217.9 |  |  |  | $374.7 |  |  |  | $394.5 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012
Cost of removal obligations |  | $411.3 |  |  |  | $408.7 |  |  |  | $270.6 |  |  |  | $268.0 |  |  |  | $140.7 |  |  |  | $140.7 |
IPL’s tax benefit riders | 335.1 |  |  | 355.8 |  |  | 335.1 |  |  | 355.8 |  |  | — |  |  | — |
Energy conservation cost recovery | 61.3 |  |  | 55.1 |  |  | 16.4 |  |  | 10.0 |  |  | 44.9 |  |  | 45.1 |
IPL’s electric transmission assets sale | 29.7 |  |  | 32.5 |  |  | 29.7 |  |  | 32.5 |  |  | — |  |  | — |
Commodity cost recovery | 39.2 |  |  | 17.7 |  |  | 25.4 |  |  | 5.2 |  |  | 13.8 |  |  | 12.5 |
Other | 59.1 |  |  | 46.3 |  |  | 38.0 |  |  | 29.9 |  |  | 21.1 |  |  | 16.4 |
|  | $935.7 |  |  |  | $916.1 |  |  |  | $715.2 |  |  |  | $701.4 |  |  |  | $220.5 |  |  |  | $214.7 |
IPL [Member] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012
Tax-related |  | $781.9 |  |  |  | $770.7 |  |  |  | $757.2 |  |  |  | $746.2 |  |  |  | $24.7 |  |  |  | $24.5 |
Pension and other postretirement benefits costs | 541.4 |  |  | 549.2 |  |  | 275.6 |  |  | 279.3 |  |  | 265.8 |  |  | 269.9 |
Asset retirement obligations (AROs) | 62.9 |  |  | 62.4 |  |  | 38.5 |  |  | 38.6 |  |  | 24.4 |  |  | 23.8 |
Environmental-related costs | 32.2 |  |  | 34.9 |  |  | 27.7 |  |  | 30.3 |  |  | 4.5 |  |  | 4.6 |
Emission allowances | 30.0 |  |  | 30.0 |  |  | 30.0 |  |  | 30.0 |  |  | — |  |  | — |
Derivatives | 17.0 |  |  | 40.2 |  |  | 6.3 |  |  | 16.3 |  |  | 10.7 |  |  | 23.9 |
Other | 117.2 |  |  | 125.0 |  |  | 72.6 |  |  | 77.2 |  |  | 44.6 |  |  | 47.8 |
|  | $1,582.6 |  |  |  | $1,612.4 |  |  |  | $1,207.9 |  |  |  | $1,217.9 |  |  |  | $374.7 |  |  |  | $394.5 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012
Cost of removal obligations |  | $411.3 |  |  |  | $408.7 |  |  |  | $270.6 |  |  |  | $268.0 |  |  |  | $140.7 |  |  |  | $140.7 |
IPL’s tax benefit riders | 335.1 |  |  | 355.8 |  |  | 335.1 |  |  | 355.8 |  |  | — |  |  | — |
Energy conservation cost recovery | 61.3 |  |  | 55.1 |  |  | 16.4 |  |  | 10.0 |  |  | 44.9 |  |  | 45.1 |
IPL’s electric transmission assets sale | 29.7 |  |  | 32.5 |  |  | 29.7 |  |  | 32.5 |  |  | — |  |  | — |
Commodity cost recovery | 39.2 |  |  | 17.7 |  |  | 25.4 |  |  | 5.2 |  |  | 13.8 |  |  | 12.5 |
Other | 59.1 |  |  | 46.3 |  |  | 38.0 |  |  | 29.9 |  |  | 21.1 |  |  | 16.4 |
|  | $935.7 |  |  |  | $916.1 |  |  |  | $715.2 |  |  |  | $701.4 |  |  |  | $220.5 |  |  |  | $214.7 |
WPL [Member] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012
Tax-related |  | $781.9 |  |  |  | $770.7 |  |  |  | $757.2 |  |  |  | $746.2 |  |  |  | $24.7 |  |  |  | $24.5 |
Pension and other postretirement benefits costs | 541.4 |  |  | 549.2 |  |  | 275.6 |  |  | 279.3 |  |  | 265.8 |  |  | 269.9 |
Asset retirement obligations (AROs) | 62.9 |  |  | 62.4 |  |  | 38.5 |  |  | 38.6 |  |  | 24.4 |  |  | 23.8 |
Environmental-related costs | 32.2 |  |  | 34.9 |  |  | 27.7 |  |  | 30.3 |  |  | 4.5 |  |  | 4.6 |
Emission allowances | 30.0 |  |  | 30.0 |  |  | 30.0 |  |  | 30.0 |  |  | — |  |  | — |
Derivatives | 17.0 |  |  | 40.2 |  |  | 6.3 |  |  | 16.3 |  |  | 10.7 |  |  | 23.9 |
Other | 117.2 |  |  | 125.0 |  |  | 72.6 |  |  | 77.2 |  |  | 44.6 |  |  | 47.8 |
|  | $1,582.6 |  |  |  | $1,612.4 |  |  |  | $1,207.9 |  |  |  | $1,217.9 |  |  |  | $374.7 |  |  |  | $394.5 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012 |  | March 31, 2013 |  | December 31, 2012
Cost of removal obligations |  | $411.3 |  |  |  | $408.7 |  |  |  | $270.6 |  |  |  | $268.0 |  |  |  | $140.7 |  |  |  | $140.7 |
IPL’s tax benefit riders | 335.1 |  |  | 355.8 |  |  | 335.1 |  |  | 355.8 |  |  | — |  |  | — |
Energy conservation cost recovery | 61.3 |  |  | 55.1 |  |  | 16.4 |  |  | 10.0 |  |  | 44.9 |  |  | 45.1 |
IPL’s electric transmission assets sale | 29.7 |  |  | 32.5 |  |  | 29.7 |  |  | 32.5 |  |  | — |  |  | — |
Commodity cost recovery | 39.2 |  |  | 17.7 |  |  | 25.4 |  |  | 5.2 |  |  | 13.8 |  |  | 12.5 |
Other | 59.1 |  |  | 46.3 |  |  | 38.0 |  |  | 29.9 |  |  | 21.1 |  |  | 16.4 |
|  | $935.7 |  |  |  | $916.1 |  |  |  | $715.2 |  |  |  | $701.4 |  |  |  | $220.5 |  |  |  | $214.7 |

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- Definition Tabular disclosure of assets that are created when regulatory agencies permit public utilities to defer certain costs included in rate-setting to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 715 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501251&loc=d3e52485-110419 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44264-110382 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 10 -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. Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9 -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. Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition Tabular disclosure of regulatory liabilities. Detailed information about liabilities that result from rate actions of a regulator. Rate actions of a regulator can impose a liability on a regulated enterprise resulting in a regulatory liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -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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