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Income Taxes | 6 Months Ended
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Jun. 30, 2015
Income Taxes [Line Items] |
Income Taxes | INCOME TAXES Income Tax Rates - The provision for income taxes for earnings from continuing operations is based on an estimated annual effective income tax rate that excludes the impact of significant unusual or infrequently occurring items, discontinued operations or extraordinary items. The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes.
| Alliant Energy |  | IPL |  | WPL
Three Months Ended June 30 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
IPL’s tax benefit riders | (10.6 | ) |  | (11.9 | ) |  | (25.3 | ) |  | (36.6 | ) |  | — |  |  | — |
Effect of rate-making on property-related differences | (8.4 | ) |  | (4.8 | ) |  | (22.3 | ) |  | (14.4 | ) |  | (0.7 | ) |  | (0.2 | )
Production tax credits | (6.5 | ) |  | (6.1 | ) |  | (7.2 | ) |  | (8.0 | ) |  | (6.7 | ) |  | (5.8 | )
Other items, net | 4.1 |  |  | 4.8 |  |  | 4.1 |  |  | 4.6 |  |  | 4.6 |  |  | 5.0 |
Overall income tax rate | 13.6 | % |  | 17.0 | % |  | (15.7 | %) |  | (19.4 | %) |  | 32.2 | % |  | 34.0 | %
| Alliant Energy |  | IPL |  | WPL
Six Months Ended June 30 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
IPL’s tax benefit riders | (10.2 | ) |  | (11.9 | ) |  | (25.2 | ) |  | (32.8 | ) |  | — |  |  | — |
Effect of rate-making on property-related differences | (7.0 | ) |  | (5.1 | ) |  | (16.6 | ) |  | (13.0 | ) |  | (0.6 | ) |  | (0.7 | )
Production tax credits | (6.4 | ) |  | (6.5 | ) |  | (7.2 | ) |  | (8.2 | ) |  | (6.4 | ) |  | (6.2 | )
Other items, net | 4.3 |  |  | 4.7 |  |  | 4.2 |  |  | 4.6 |  |  | 4.7 |  |  | 4.9 |
Overall income tax rate | 15.7 | % |  | 16.2 | % |  | (9.8 | %) |  | (14.4 | %) |  | 32.7 | % |  | 33.0 | %
IPL’s tax benefit riders - Alliant Energy’s and IPL’s effective income tax rates include the impact of reducing income tax expense with offsetting reductions to regulatory liabilities as a result of implementing IPL’s tax benefit riders. Refer to Note 2 for additional details of the tax benefit riders. Effect of rate-making on property-related differences - Alliant Energy’s and IPL’s effective income tax rates are impacted by the effect of rate-making principles on certain property-related differences for which deferred tax is not recognized in the income statement, substantially all of which relates to IPL. The increased benefits from property-related differences recognized during the three and six months ended June 30, 2015 were primarily due to additional repairs deductions. Deferred Tax Assets and Liabilities - For the six months ended June 30, 2015, Alliant Energy’s, IPL’s and WPL’s non-current deferred tax liabilities increased $128.0 million, $77.4 million and $44.9 million, respectively. These increases in non-current deferred tax liabilities were primarily due to utilization of federal net operating loss carryforwards, and property-related differences recorded during the six months ended June 30, 2015, including an increase in qualifying repairs expenditures. Carryforwards - At June 30, 2015, tax carryforwards and associated deferred tax assets and expiration dates were estimated as follows (dollars in millions):
Alliant Energy | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $845 |  |  |  | $290 |  |  | 2029
State net operating losses | 860 |  |  | 44 |  |  | 2018
Federal tax credits | 222 |  |  | 218 |  |  | 2022
|  |  |  | $552 |  |  |
IPL | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $396 |  |  |  | $136 |  |  | 2029
State net operating losses | 265 |  |  | 14 |  |  | 2018
Federal tax credits | 77 |  |  | 75 |  |  | 2022
|  |  |  | $225 |  |  |
WPL | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $342 |  |  |  | $117 |  |  | 2029
State net operating losses | 170 |  |  | 8 |  |  | 2018
Federal tax credits | 86 |  |  | 85 |  |  | 2022
|  |  |  | $210 |  |  |
IPL [Member] |
Income Taxes [Line Items] |
Income Taxes | INCOME TAXES Income Tax Rates - The provision for income taxes for earnings from continuing operations is based on an estimated annual effective income tax rate that excludes the impact of significant unusual or infrequently occurring items, discontinued operations or extraordinary items. The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes.
| Alliant Energy |  | IPL |  | WPL
Three Months Ended June 30 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
IPL’s tax benefit riders | (10.6 | ) |  | (11.9 | ) |  | (25.3 | ) |  | (36.6 | ) |  | — |  |  | — |
Effect of rate-making on property-related differences | (8.4 | ) |  | (4.8 | ) |  | (22.3 | ) |  | (14.4 | ) |  | (0.7 | ) |  | (0.2 | )
Production tax credits | (6.5 | ) |  | (6.1 | ) |  | (7.2 | ) |  | (8.0 | ) |  | (6.7 | ) |  | (5.8 | )
Other items, net | 4.1 |  |  | 4.8 |  |  | 4.1 |  |  | 4.6 |  |  | 4.6 |  |  | 5.0 |
Overall income tax rate | 13.6 | % |  | 17.0 | % |  | (15.7 | %) |  | (19.4 | %) |  | 32.2 | % |  | 34.0 | %
| Alliant Energy |  | IPL |  | WPL
Six Months Ended June 30 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
IPL’s tax benefit riders | (10.2 | ) |  | (11.9 | ) |  | (25.2 | ) |  | (32.8 | ) |  | — |  |  | — |
Effect of rate-making on property-related differences | (7.0 | ) |  | (5.1 | ) |  | (16.6 | ) |  | (13.0 | ) |  | (0.6 | ) |  | (0.7 | )
Production tax credits | (6.4 | ) |  | (6.5 | ) |  | (7.2 | ) |  | (8.2 | ) |  | (6.4 | ) |  | (6.2 | )
Other items, net | 4.3 |  |  | 4.7 |  |  | 4.2 |  |  | 4.6 |  |  | 4.7 |  |  | 4.9 |
Overall income tax rate | 15.7 | % |  | 16.2 | % |  | (9.8 | %) |  | (14.4 | %) |  | 32.7 | % |  | 33.0 | %
IPL’s tax benefit riders - Alliant Energy’s and IPL’s effective income tax rates include the impact of reducing income tax expense with offsetting reductions to regulatory liabilities as a result of implementing IPL’s tax benefit riders. Refer to Note 2 for additional details of the tax benefit riders. Effect of rate-making on property-related differences - Alliant Energy’s and IPL’s effective income tax rates are impacted by the effect of rate-making principles on certain property-related differences for which deferred tax is not recognized in the income statement, substantially all of which relates to IPL. The increased benefits from property-related differences recognized during the three and six months ended June 30, 2015 were primarily due to additional repairs deductions. Deferred Tax Assets and Liabilities - For the six months ended June 30, 2015, Alliant Energy’s, IPL’s and WPL’s non-current deferred tax liabilities increased $128.0 million, $77.4 million and $44.9 million, respectively. These increases in non-current deferred tax liabilities were primarily due to utilization of federal net operating loss carryforwards, and property-related differences recorded during the six months ended June 30, 2015, including an increase in qualifying repairs expenditures. Carryforwards - At June 30, 2015, tax carryforwards and associated deferred tax assets and expiration dates were estimated as follows (dollars in millions):
Alliant Energy | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $845 |  |  |  | $290 |  |  | 2029
State net operating losses | 860 |  |  | 44 |  |  | 2018
Federal tax credits | 222 |  |  | 218 |  |  | 2022
|  |  |  | $552 |  |  |
IPL | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $396 |  |  |  | $136 |  |  | 2029
State net operating losses | 265 |  |  | 14 |  |  | 2018
Federal tax credits | 77 |  |  | 75 |  |  | 2022
|  |  |  | $225 |  |  |
WPL | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $342 |  |  |  | $117 |  |  | 2029
State net operating losses | 170 |  |  | 8 |  |  | 2018
Federal tax credits | 86 |  |  | 85 |  |  | 2022
|  |  |  | $210 |  |  |
WPL [Member] |
Income Taxes [Line Items] |
Income Taxes | INCOME TAXES Income Tax Rates - The provision for income taxes for earnings from continuing operations is based on an estimated annual effective income tax rate that excludes the impact of significant unusual or infrequently occurring items, discontinued operations or extraordinary items. The overall income tax rates shown in the following table were computed by dividing income tax expense (benefit) by income from continuing operations before income taxes.
| Alliant Energy |  | IPL |  | WPL
Three Months Ended June 30 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
IPL’s tax benefit riders | (10.6 | ) |  | (11.9 | ) |  | (25.3 | ) |  | (36.6 | ) |  | — |  |  | — |
Effect of rate-making on property-related differences | (8.4 | ) |  | (4.8 | ) |  | (22.3 | ) |  | (14.4 | ) |  | (0.7 | ) |  | (0.2 | )
Production tax credits | (6.5 | ) |  | (6.1 | ) |  | (7.2 | ) |  | (8.0 | ) |  | (6.7 | ) |  | (5.8 | )
Other items, net | 4.1 |  |  | 4.8 |  |  | 4.1 |  |  | 4.6 |  |  | 4.6 |  |  | 5.0 |
Overall income tax rate | 13.6 | % |  | 17.0 | % |  | (15.7 | %) |  | (19.4 | %) |  | 32.2 | % |  | 34.0 | %
| Alliant Energy |  | IPL |  | WPL
Six Months Ended June 30 | 2015 |  | 2014 |  | 2015 |  | 2014 |  | 2015 |  | 2014
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
IPL’s tax benefit riders | (10.2 | ) |  | (11.9 | ) |  | (25.2 | ) |  | (32.8 | ) |  | — |  |  | — |
Effect of rate-making on property-related differences | (7.0 | ) |  | (5.1 | ) |  | (16.6 | ) |  | (13.0 | ) |  | (0.6 | ) |  | (0.7 | )
Production tax credits | (6.4 | ) |  | (6.5 | ) |  | (7.2 | ) |  | (8.2 | ) |  | (6.4 | ) |  | (6.2 | )
Other items, net | 4.3 |  |  | 4.7 |  |  | 4.2 |  |  | 4.6 |  |  | 4.7 |  |  | 4.9 |
Overall income tax rate | 15.7 | % |  | 16.2 | % |  | (9.8 | %) |  | (14.4 | %) |  | 32.7 | % |  | 33.0 | %
IPL’s tax benefit riders - Alliant Energy’s and IPL’s effective income tax rates include the impact of reducing income tax expense with offsetting reductions to regulatory liabilities as a result of implementing IPL’s tax benefit riders. Refer to Note 2 for additional details of the tax benefit riders. Effect of rate-making on property-related differences - Alliant Energy’s and IPL’s effective income tax rates are impacted by the effect of rate-making principles on certain property-related differences for which deferred tax is not recognized in the income statement, substantially all of which relates to IPL. The increased benefits from property-related differences recognized during the three and six months ended June 30, 2015 were primarily due to additional repairs deductions. Deferred Tax Assets and Liabilities - For the six months ended June 30, 2015, Alliant Energy’s, IPL’s and WPL’s non-current deferred tax liabilities increased $128.0 million, $77.4 million and $44.9 million, respectively. These increases in non-current deferred tax liabilities were primarily due to utilization of federal net operating loss carryforwards, and property-related differences recorded during the six months ended June 30, 2015, including an increase in qualifying repairs expenditures. Carryforwards - At June 30, 2015, tax carryforwards and associated deferred tax assets and expiration dates were estimated as follows (dollars in millions):
Alliant Energy | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $845 |  |  |  | $290 |  |  | 2029
State net operating losses | 860 |  |  | 44 |  |  | 2018
Federal tax credits | 222 |  |  | 218 |  |  | 2022
|  |  |  | $552 |  |  |
IPL | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $396 |  |  |  | $136 |  |  | 2029
State net operating losses | 265 |  |  | 14 |  |  | 2018
Federal tax credits | 77 |  |  | 75 |  |  | 2022
|  |  |  | $225 |  |  |
WPL | Tax Carryforwards |  | Deferred Tax Assets |  | Earliest Expiration Date
Federal net operating losses |  | $342 |  |  |  | $117 |  |  | 2029
State net operating losses | 170 |  |  | 8 |  |  | 2018
Federal tax credits | 86 |  |  | 85 |  |  | 2022
|  |  |  | $210 |  |  |

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