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Income Taxes | 12 Months Ended
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Dec. 31, 2015
Income Tax [Line Items] |
Income Taxes | INCOME TAXES Refer to Note 1(c) for discussion of a change implemented in the fourth quarter of 2015 related to IPL’s and WPL’s method of recording income taxes, which was applied retrospectively. Refer to Note 1(p) for discussion of the adoption of an accounting standard to simplify the presentation of deferred taxes on the balance sheet, which was applied prospectively. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013
Current tax expense (benefit): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Federal |  | $2.0 |  |  |  | $36.6 |  |  |  | $4.4 |  |  |  | ($14.1 | ) |  |  | $8.9 |  |  |  | $10.2 |  |  |  | $4.7 |  |  |  | $2.0 |  |  |  | ($0.3 | )
State | 3.2 |  |  | 9.3 |  |  | (3.6 | ) |  | 11.5 |  |  | 10.4 |  |  | 4.2 |  |  | 0.6 |  |  | 0.8 |  |  | (4.3 | )
IPL’s tax benefit riders | (49.0 | ) |  | (56.7 | ) |  | (52.9 | ) |  | (49.0 | ) |  | (56.7 | ) |  | (52.9 | ) |  | — |  |  | — |  |  | — |
Deferred tax expense (benefit): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Federal | 120.8 |  |  | 83.5 |  |  | 123.9 |  |  | 40.7 |  |  | 10.8 |  |  | 21.7 |  |  | 76.8 |  |  | 81.1 |  |  | 84.0 |
State | 27.9 |  |  | 4.6 |  |  | 15.6 |  |  | 3.3 |  |  | (7.9 | ) |  | (4.0 | ) |  | 20.2 |  |  | 20.0 |  |  | 24.1 |
Production tax credits | (33.1 | ) |  | (31.3 | ) |  | (31.0 | ) |  | (14.5 | ) |  | (13.8 | ) |  | (14.1 | ) |  | (18.6 | ) |  | (17.5 | ) |  | (16.9 | )
Investment tax credits | (1.4 | ) |  | (1.6 | ) |  | (1.6 | ) |  | (0.6 | ) |  | (0.6 | ) |  | (0.6 | ) |  | (0.8 | ) |  | (1.0 | ) |  | (1.0 | )
Provision recorded as a change in uncertain tax positions: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred | — |  |  | — |  |  | (0.4 | ) |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | (0.4 | )
Provision recorded as a change in accrued interest | — |  |  | (0.1 | ) |  | (0.5 | ) |  | — |  |  | — |  |  | (0.8 | ) |  | — |  |  | (0.1 | ) |  | 0.4 |
|  | $70.4 |  |  |  | $44.3 |  |  |  | $53.9 |  |  |  | ($22.7 | ) |  |  | ($48.9 | ) |  |  | ($36.3 | ) |  |  | $82.9 |  |  |  | $85.3 |  |  |  | $85.6 |
Income Tax Rates - 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
| 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
State income taxes, net of federal benefits | 5.2 |  |  | 5.4 |  |  | 5.7 |  |  | 6.2 |  |  | 6.1 |  |  | 6.3 |  |  | 5.1 |  |  | 5.1 |  |  | 5.5 |
IPL’s tax benefit riders | (10.6 | ) |  | (12.9 | ) |  | (12.1 | ) |  | (28.3 | ) |  | (39.6 | ) |  | (34.8 | ) |  | — |  |  | — |  |  | — |
Effect of rate-making on property-related differences | (6.8 | ) |  | (7.5 | ) |  | (6.0 | ) |  | (17.2 | ) |  | (21.9 | ) |  | (15.9 | ) |  | (0.5 | ) |  | (0.7 | ) |  | (0.8 | )
Production tax credits | (7.2 | ) |  | (7.1 | ) |  | (7.1 | ) |  | (8.3 | ) |  | (9.6 | ) |  | (9.3 | ) |  | (7.1 | ) |  | (6.6 | ) |  | (6.4 | )
Adjustment of prior period taxes | 0.8 |  |  | (1.3 | ) |  | (1.3 | ) |  | 0.7 |  |  | (3.0 | ) |  | (3.6 | ) |  | 0.1 |  |  | — |  |  | (0.2 | )
Other items, net | (1.1 | ) |  | (1.5 | ) |  | (1.8 | ) |  | (1.2 | ) |  | (1.2 | ) |  | (1.6 | ) |  | (0.8 | ) |  | (0.8 | ) |  | (0.8 | )
Overall income tax rate | 15.3 | % |  | 10.1 | % |  | 12.4 | % |  | (13.1 | %) |  | (34.2 | %) |  | (23.9 | %) |  | 31.8 | % |  | 32.0 | % |  | 32.3 | %
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 income tax expense and benefits are impacted by certain property-related differences at IPL for which deferred tax is not recognized in the income statement pursuant to Iowa rate-making principles. In 2014, the increased benefits from property-related differences were primarily due to additional repairs deductions and additional deductions from the allocation of mixed service costs related to Marshalltown. Production tax credits - Production tax credits are earned from owned and operated wind farms. Production tax credits are based on the electricity generated by each wind farm during the first 10 years of operation. Details regarding production tax credits (net of state tax impacts) related to various wind farms are as follows (dollars in millions):
| End of Production |  | Nameplate |  |  |  |  |  |
| Tax Credit Generation |  | Capacity in MW |  | 2015 |  | 2014 |  | 2013
Cedar Ridge (WPL) | December 2018 |  | 68 |  |  | $4.5 |  |  |  | $4.1 |  |  |  | $4.2 |
Bent Tree (WPL) | February 2021 |  | 201 |  | 14.1 |  |  | 13.4 |  |  | 12.7 |
Subtotal (WPL) |  |  |  |  | 18.6 |  |  | 17.5 |  |  | 16.9 |
Whispering Willow - East (IPL) | December 2019 |  | 200 |  | 14.5 |  |  | 13.8 |  |  | 14.1 |
|  |  |  |  |  | $33.1 |  |  |  | $31.3 |  |  |  | $31.0 |
Deferred Tax Assets and Liabilities - The deferred tax (assets) and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions):
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
Alliant Energy | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $2,762.9 |  |  | $2,762.9 |  |  |  | $— |  |  | $2,627.8 |  |  | $2,627.8 |
Investment in ATC | — |  | 138.1 |  | 138.1 |  |  | — |  | 131.6 |  | 131.6 |
Net operating losses carryforwards - state | (38.3 | ) | — |  | (38.3 | ) |  | (45.7 | ) | — |  | (45.7 | )
Regulatory liability - IPL’s tax benefit riders | (66.1 | ) | — |  | (66.1 | ) |  | (100.9 | ) | — |  | (100.9 | )
Federal credit carryforwards | (236.4 | ) | — |  | (236.4 | ) |  | (201.0 | ) | — |  | (201.0 | )
Net operating losses carryforwards - federal | (250.9 | ) | — |  | (250.9 | ) |  | (332.8 | ) | — |  | (332.8 | )
Other | (85.4 | ) | 157.3 |  | 71.9 |  |  | (88.1 | ) | 180.1 |  | 92.0 |
|  | ($677.1 | ) |  | $3,058.3 |  |  | $2,381.2 |  |  |  | ($768.5 | ) |  | $2,939.5 |  |  | $2,171.0 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($150.1 | )
Non-current deferred tax liabilities |  | $2,381.2 |  |  | 2,321.1 |
Total net deferred tax liabilities |  | $2,381.2 |  |  |  | $2,171.0 |
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
IPL | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $1,587.8 |  |  | $1,587.8 |  |  |  | $— |  |  | $1,545.1 |  |  | $1,545.1 |
Regulatory liability - tax benefit riders | (66.1 | ) | — |  | (66.1 | ) |  | (100.9 | ) | — |  | (100.9 | )
Federal credit carryforwards | (81.7 | ) | — |  | (81.7 | ) |  | (66.5 | ) | — |  | (66.5 | )
Net operating losses carryforwards - federal | (113.1 | ) | — |  | (113.1 | ) |  | (157.8 | ) | — |  | (157.8 | )
Other | (36.7 | ) | 87.8 |  | 51.1 |  |  | (37.1 | ) | 83.5 |  | 46.4 |
|  | ($297.6 | ) |  | $1,675.6 |  |  | $1,378.0 |  |  |  | ($362.3 | ) |  | $1,628.6 |  |  | $1,266.3 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($104.0 | )
Non-current deferred tax liabilities |  | $1,378.0 |  |  | 1,370.3 |
Total net deferred tax liabilities |  | $1,378.0 |  |  |  | $1,266.3 |
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
WPL | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $1,027.0 |  |  | $1,027.0 |  |  |  | $— |  |  | $963.0 |  |  | $963.0 |
Investment in ATC | — |  | 138.9 |  | 138.9 |  |  | — |  | 132.1 |  | 132.1 |
Federal credit carryforwards | (95.5 | ) | — |  | (95.5 | ) |  | (76.7 | ) | — |  | (76.7 | )
Net operating losses carryforwards - federal | (105.1 | ) | — |  | (105.1 | ) |  | (131.6 | ) | — |  | (131.6 | )
Other | (27.8 | ) | 67.9 |  | 40.1 |  |  | (37.1 | ) | 79.7 |  | 42.6 |
|  | ($228.4 | ) |  | $1,233.8 |  |  | $1,005.4 |  |  |  | ($245.4 | ) |  | $1,174.8 |  |  | $929.4 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($37.7 | )
Non-current deferred tax liabilities |  | $1,005.4 |  |  | 967.1 |
Total net deferred tax liabilities |  | $1,005.4 |  |  |  | $929.4 |
Property - Property-related differences were primarily related to accelerated depreciation, including bonus depreciation. In December 2015, the PATH Act was enacted. The most significant provisions of the PATH Act for Alliant Energy, IPL and WPL relate to the extension of bonus depreciation deductions for certain expenditures for property incurred through December 31, 2019 and placed in service prior to December 31, 2020. Alliant Energy currently estimates its total bonus depreciation deductions to be claimed on its U.S. federal income tax return for calendar year 2015 will be approximately $200 million ($100 million for IPL and $50 million for WPL). Investment in ATC - WPL Transco has a partial ownership interest in ATC, which has generated deferred tax liabilities primarily from tax depreciation deductions taken at ATC in excess of book depreciation. The increase in deferred tax liabilities in 2015 was primarily due to bonus depreciation deductions estimated at ATC. Regulatory liability tax benefit riders - Refer to Note 2 for discussion of regulatory liabilities associated with IPL’s tax benefit riders. Carryforwards - At December 31, 2015, tax carryforwards and associated deferred tax assets and expiration dates were estimated as follows (in millions):
|  |  | Alliant Energy |  | IPL |  | WPL
| Earliest |  | Tax |  | Deferred |  | Tax |  | Deferred |  | Tax |  | Deferred
| Expiration Date |  | Carryforwards |  | Tax Assets |  | Carryforwards |  | Tax Assets |  | Carryforwards |  | Tax Assets
Federal net operating losses | 2030 |  |  | $732 |  |  |  | $251 |  |  |  | $336 |  |  |  | $113 |  |  |  | $300 |  |  |  | $105 |
State net operating losses | 2018 |  | 766 |  |  | 38 |  |  | 20 |  |  | 1 |  |  | 76 |  |  | 4 |
Federal tax credits | 2022 |  | 240 |  |  | 236 |  |  | 85 |  |  | 82 |  |  | 96 |  |  | 96 |
|  |  |  |  |  | $525 |  |  |  |  |  | $196 |  |  |  |  |  | $205 |
At December 31, 2015, Alliant Energy’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 99% expiring after 2024. At December 31, 2015, IPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 95% expiring after 2021. At December 31, 2015, WPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 96% expiring after 2033. Uncertain Tax Positions - At December 31, 2015, 2014 and 2013, there were no uncertain tax positions or penalties accrued related to uncertain tax positions, and interest accrued and tax positions favorably impacting future effective tax rates for continuing operations were not material. As of December 31, 2015, no material changes to unrecognized tax benefits are expected during the next 12 months. Open tax years - Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows:
Consolidated federal income tax returns (a) | 2012 | - | 2014
Consolidated Iowa income tax returns (b) | 2012 | - | 2014
Wisconsin combined tax returns (c) | 2011 | - | 2014
(a) | These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date.
(b) | The statute of limitations for these Iowa tax returns expires three years from each filing date.
(c) | The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.
IPL [Member] |
Income Tax [Line Items] |
Income Taxes | INCOME TAXES Refer to Note 1(c) for discussion of a change implemented in the fourth quarter of 2015 related to IPL’s and WPL’s method of recording income taxes, which was applied retrospectively. Refer to Note 1(p) for discussion of the adoption of an accounting standard to simplify the presentation of deferred taxes on the balance sheet, which was applied prospectively. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013
Current tax expense (benefit): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Federal |  | $2.0 |  |  |  | $36.6 |  |  |  | $4.4 |  |  |  | ($14.1 | ) |  |  | $8.9 |  |  |  | $10.2 |  |  |  | $4.7 |  |  |  | $2.0 |  |  |  | ($0.3 | )
State | 3.2 |  |  | 9.3 |  |  | (3.6 | ) |  | 11.5 |  |  | 10.4 |  |  | 4.2 |  |  | 0.6 |  |  | 0.8 |  |  | (4.3 | )
IPL’s tax benefit riders | (49.0 | ) |  | (56.7 | ) |  | (52.9 | ) |  | (49.0 | ) |  | (56.7 | ) |  | (52.9 | ) |  | — |  |  | — |  |  | — |
Deferred tax expense (benefit): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Federal | 120.8 |  |  | 83.5 |  |  | 123.9 |  |  | 40.7 |  |  | 10.8 |  |  | 21.7 |  |  | 76.8 |  |  | 81.1 |  |  | 84.0 |
State | 27.9 |  |  | 4.6 |  |  | 15.6 |  |  | 3.3 |  |  | (7.9 | ) |  | (4.0 | ) |  | 20.2 |  |  | 20.0 |  |  | 24.1 |
Production tax credits | (33.1 | ) |  | (31.3 | ) |  | (31.0 | ) |  | (14.5 | ) |  | (13.8 | ) |  | (14.1 | ) |  | (18.6 | ) |  | (17.5 | ) |  | (16.9 | )
Investment tax credits | (1.4 | ) |  | (1.6 | ) |  | (1.6 | ) |  | (0.6 | ) |  | (0.6 | ) |  | (0.6 | ) |  | (0.8 | ) |  | (1.0 | ) |  | (1.0 | )
Provision recorded as a change in uncertain tax positions: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred | — |  |  | — |  |  | (0.4 | ) |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | (0.4 | )
Provision recorded as a change in accrued interest | — |  |  | (0.1 | ) |  | (0.5 | ) |  | — |  |  | — |  |  | (0.8 | ) |  | — |  |  | (0.1 | ) |  | 0.4 |
|  | $70.4 |  |  |  | $44.3 |  |  |  | $53.9 |  |  |  | ($22.7 | ) |  |  | ($48.9 | ) |  |  | ($36.3 | ) |  |  | $82.9 |  |  |  | $85.3 |  |  |  | $85.6 |
Income Tax Rates - 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
| 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
State income taxes, net of federal benefits | 5.2 |  |  | 5.4 |  |  | 5.7 |  |  | 6.2 |  |  | 6.1 |  |  | 6.3 |  |  | 5.1 |  |  | 5.1 |  |  | 5.5 |
IPL’s tax benefit riders | (10.6 | ) |  | (12.9 | ) |  | (12.1 | ) |  | (28.3 | ) |  | (39.6 | ) |  | (34.8 | ) |  | — |  |  | — |  |  | — |
Effect of rate-making on property-related differences | (6.8 | ) |  | (7.5 | ) |  | (6.0 | ) |  | (17.2 | ) |  | (21.9 | ) |  | (15.9 | ) |  | (0.5 | ) |  | (0.7 | ) |  | (0.8 | )
Production tax credits | (7.2 | ) |  | (7.1 | ) |  | (7.1 | ) |  | (8.3 | ) |  | (9.6 | ) |  | (9.3 | ) |  | (7.1 | ) |  | (6.6 | ) |  | (6.4 | )
Adjustment of prior period taxes | 0.8 |  |  | (1.3 | ) |  | (1.3 | ) |  | 0.7 |  |  | (3.0 | ) |  | (3.6 | ) |  | 0.1 |  |  | — |  |  | (0.2 | )
Other items, net | (1.1 | ) |  | (1.5 | ) |  | (1.8 | ) |  | (1.2 | ) |  | (1.2 | ) |  | (1.6 | ) |  | (0.8 | ) |  | (0.8 | ) |  | (0.8 | )
Overall income tax rate | 15.3 | % |  | 10.1 | % |  | 12.4 | % |  | (13.1 | %) |  | (34.2 | %) |  | (23.9 | %) |  | 31.8 | % |  | 32.0 | % |  | 32.3 | %
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 income tax expense and benefits are impacted by certain property-related differences at IPL for which deferred tax is not recognized in the income statement pursuant to Iowa rate-making principles. In 2014, the increased benefits from property-related differences were primarily due to additional repairs deductions and additional deductions from the allocation of mixed service costs related to Marshalltown. Production tax credits - Production tax credits are earned from owned and operated wind farms. Production tax credits are based on the electricity generated by each wind farm during the first 10 years of operation. Details regarding production tax credits (net of state tax impacts) related to various wind farms are as follows (dollars in millions):
| End of Production |  | Nameplate |  |  |  |  |  |
| Tax Credit Generation |  | Capacity in MW |  | 2015 |  | 2014 |  | 2013
Cedar Ridge (WPL) | December 2018 |  | 68 |  |  | $4.5 |  |  |  | $4.1 |  |  |  | $4.2 |
Bent Tree (WPL) | February 2021 |  | 201 |  | 14.1 |  |  | 13.4 |  |  | 12.7 |
Subtotal (WPL) |  |  |  |  | 18.6 |  |  | 17.5 |  |  | 16.9 |
Whispering Willow - East (IPL) | December 2019 |  | 200 |  | 14.5 |  |  | 13.8 |  |  | 14.1 |
|  |  |  |  |  | $33.1 |  |  |  | $31.3 |  |  |  | $31.0 |
Deferred Tax Assets and Liabilities - The deferred tax (assets) and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions):
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
Alliant Energy | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $2,762.9 |  |  | $2,762.9 |  |  |  | $— |  |  | $2,627.8 |  |  | $2,627.8 |
Investment in ATC | — |  | 138.1 |  | 138.1 |  |  | — |  | 131.6 |  | 131.6 |
Net operating losses carryforwards - state | (38.3 | ) | — |  | (38.3 | ) |  | (45.7 | ) | — |  | (45.7 | )
Regulatory liability - IPL’s tax benefit riders | (66.1 | ) | — |  | (66.1 | ) |  | (100.9 | ) | — |  | (100.9 | )
Federal credit carryforwards | (236.4 | ) | — |  | (236.4 | ) |  | (201.0 | ) | — |  | (201.0 | )
Net operating losses carryforwards - federal | (250.9 | ) | — |  | (250.9 | ) |  | (332.8 | ) | — |  | (332.8 | )
Other | (85.4 | ) | 157.3 |  | 71.9 |  |  | (88.1 | ) | 180.1 |  | 92.0 |
|  | ($677.1 | ) |  | $3,058.3 |  |  | $2,381.2 |  |  |  | ($768.5 | ) |  | $2,939.5 |  |  | $2,171.0 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($150.1 | )
Non-current deferred tax liabilities |  | $2,381.2 |  |  | 2,321.1 |
Total net deferred tax liabilities |  | $2,381.2 |  |  |  | $2,171.0 |
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
IPL | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $1,587.8 |  |  | $1,587.8 |  |  |  | $— |  |  | $1,545.1 |  |  | $1,545.1 |
Regulatory liability - tax benefit riders | (66.1 | ) | — |  | (66.1 | ) |  | (100.9 | ) | — |  | (100.9 | )
Federal credit carryforwards | (81.7 | ) | — |  | (81.7 | ) |  | (66.5 | ) | — |  | (66.5 | )
Net operating losses carryforwards - federal | (113.1 | ) | — |  | (113.1 | ) |  | (157.8 | ) | — |  | (157.8 | )
Other | (36.7 | ) | 87.8 |  | 51.1 |  |  | (37.1 | ) | 83.5 |  | 46.4 |
|  | ($297.6 | ) |  | $1,675.6 |  |  | $1,378.0 |  |  |  | ($362.3 | ) |  | $1,628.6 |  |  | $1,266.3 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($104.0 | )
Non-current deferred tax liabilities |  | $1,378.0 |  |  | 1,370.3 |
Total net deferred tax liabilities |  | $1,378.0 |  |  |  | $1,266.3 |
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
WPL | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $1,027.0 |  |  | $1,027.0 |  |  |  | $— |  |  | $963.0 |  |  | $963.0 |
Investment in ATC | — |  | 138.9 |  | 138.9 |  |  | — |  | 132.1 |  | 132.1 |
Federal credit carryforwards | (95.5 | ) | — |  | (95.5 | ) |  | (76.7 | ) | — |  | (76.7 | )
Net operating losses carryforwards - federal | (105.1 | ) | — |  | (105.1 | ) |  | (131.6 | ) | — |  | (131.6 | )
Other | (27.8 | ) | 67.9 |  | 40.1 |  |  | (37.1 | ) | 79.7 |  | 42.6 |
|  | ($228.4 | ) |  | $1,233.8 |  |  | $1,005.4 |  |  |  | ($245.4 | ) |  | $1,174.8 |  |  | $929.4 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($37.7 | )
Non-current deferred tax liabilities |  | $1,005.4 |  |  | 967.1 |
Total net deferred tax liabilities |  | $1,005.4 |  |  |  | $929.4 |
Property - Property-related differences were primarily related to accelerated depreciation, including bonus depreciation. In December 2015, the PATH Act was enacted. The most significant provisions of the PATH Act for Alliant Energy, IPL and WPL relate to the extension of bonus depreciation deductions for certain expenditures for property incurred through December 31, 2019 and placed in service prior to December 31, 2020. Alliant Energy currently estimates its total bonus depreciation deductions to be claimed on its U.S. federal income tax return for calendar year 2015 will be approximately $200 million ($100 million for IPL and $50 million for WPL). Investment in ATC - WPL Transco has a partial ownership interest in ATC, which has generated deferred tax liabilities primarily from tax depreciation deductions taken at ATC in excess of book depreciation. The increase in deferred tax liabilities in 2015 was primarily due to bonus depreciation deductions estimated at ATC. Regulatory liability tax benefit riders - Refer to Note 2 for discussion of regulatory liabilities associated with IPL’s tax benefit riders. Carryforwards - At December 31, 2015, tax carryforwards and associated deferred tax assets and expiration dates were estimated as follows (in millions):
|  |  | Alliant Energy |  | IPL |  | WPL
| Earliest |  | Tax |  | Deferred |  | Tax |  | Deferred |  | Tax |  | Deferred
| Expiration Date |  | Carryforwards |  | Tax Assets |  | Carryforwards |  | Tax Assets |  | Carryforwards |  | Tax Assets
Federal net operating losses | 2030 |  |  | $732 |  |  |  | $251 |  |  |  | $336 |  |  |  | $113 |  |  |  | $300 |  |  |  | $105 |
State net operating losses | 2018 |  | 766 |  |  | 38 |  |  | 20 |  |  | 1 |  |  | 76 |  |  | 4 |
Federal tax credits | 2022 |  | 240 |  |  | 236 |  |  | 85 |  |  | 82 |  |  | 96 |  |  | 96 |
|  |  |  |  |  | $525 |  |  |  |  |  | $196 |  |  |  |  |  | $205 |
At December 31, 2015, Alliant Energy’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 99% expiring after 2024. At December 31, 2015, IPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 95% expiring after 2021. At December 31, 2015, WPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 96% expiring after 2033. Uncertain Tax Positions - At December 31, 2015, 2014 and 2013, there were no uncertain tax positions or penalties accrued related to uncertain tax positions, and interest accrued and tax positions favorably impacting future effective tax rates for continuing operations were not material. As of December 31, 2015, no material changes to unrecognized tax benefits are expected during the next 12 months. Open tax years - Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows:
Consolidated federal income tax returns (a) | 2012 | - | 2014
Consolidated Iowa income tax returns (b) | 2012 | - | 2014
Wisconsin combined tax returns (c) | 2011 | - | 2014
(a) | These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date.
(b) | The statute of limitations for these Iowa tax returns expires three years from each filing date.
(c) | The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.
WPL [Member] |
Income Tax [Line Items] |
Income Taxes | INCOME TAXES Refer to Note 1(c) for discussion of a change implemented in the fourth quarter of 2015 related to IPL’s and WPL’s method of recording income taxes, which was applied retrospectively. Refer to Note 1(p) for discussion of the adoption of an accounting standard to simplify the presentation of deferred taxes on the balance sheet, which was applied prospectively. Income Tax Expense (Benefit) - The components of “Income tax expense (benefit)” in the income statements were as follows (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013
Current tax expense (benefit): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Federal |  | $2.0 |  |  |  | $36.6 |  |  |  | $4.4 |  |  |  | ($14.1 | ) |  |  | $8.9 |  |  |  | $10.2 |  |  |  | $4.7 |  |  |  | $2.0 |  |  |  | ($0.3 | )
State | 3.2 |  |  | 9.3 |  |  | (3.6 | ) |  | 11.5 |  |  | 10.4 |  |  | 4.2 |  |  | 0.6 |  |  | 0.8 |  |  | (4.3 | )
IPL’s tax benefit riders | (49.0 | ) |  | (56.7 | ) |  | (52.9 | ) |  | (49.0 | ) |  | (56.7 | ) |  | (52.9 | ) |  | — |  |  | — |  |  | — |
Deferred tax expense (benefit): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Federal | 120.8 |  |  | 83.5 |  |  | 123.9 |  |  | 40.7 |  |  | 10.8 |  |  | 21.7 |  |  | 76.8 |  |  | 81.1 |  |  | 84.0 |
State | 27.9 |  |  | 4.6 |  |  | 15.6 |  |  | 3.3 |  |  | (7.9 | ) |  | (4.0 | ) |  | 20.2 |  |  | 20.0 |  |  | 24.1 |
Production tax credits | (33.1 | ) |  | (31.3 | ) |  | (31.0 | ) |  | (14.5 | ) |  | (13.8 | ) |  | (14.1 | ) |  | (18.6 | ) |  | (17.5 | ) |  | (16.9 | )
Investment tax credits | (1.4 | ) |  | (1.6 | ) |  | (1.6 | ) |  | (0.6 | ) |  | (0.6 | ) |  | (0.6 | ) |  | (0.8 | ) |  | (1.0 | ) |  | (1.0 | )
Provision recorded as a change in uncertain tax positions: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred | — |  |  | — |  |  | (0.4 | ) |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | (0.4 | )
Provision recorded as a change in accrued interest | — |  |  | (0.1 | ) |  | (0.5 | ) |  | — |  |  | — |  |  | (0.8 | ) |  | — |  |  | (0.1 | ) |  | 0.4 |
|  | $70.4 |  |  |  | $44.3 |  |  |  | $53.9 |  |  |  | ($22.7 | ) |  |  | ($48.9 | ) |  |  | ($36.3 | ) |  |  | $82.9 |  |  |  | $85.3 |  |  |  | $85.6 |
Income Tax Rates - 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
| 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013 |  | 2015 |  | 2014 |  | 2013
Statutory federal income tax rate | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | % |  | 35.0 | %
State income taxes, net of federal benefits | 5.2 |  |  | 5.4 |  |  | 5.7 |  |  | 6.2 |  |  | 6.1 |  |  | 6.3 |  |  | 5.1 |  |  | 5.1 |  |  | 5.5 |
IPL’s tax benefit riders | (10.6 | ) |  | (12.9 | ) |  | (12.1 | ) |  | (28.3 | ) |  | (39.6 | ) |  | (34.8 | ) |  | — |  |  | — |  |  | — |
Effect of rate-making on property-related differences | (6.8 | ) |  | (7.5 | ) |  | (6.0 | ) |  | (17.2 | ) |  | (21.9 | ) |  | (15.9 | ) |  | (0.5 | ) |  | (0.7 | ) |  | (0.8 | )
Production tax credits | (7.2 | ) |  | (7.1 | ) |  | (7.1 | ) |  | (8.3 | ) |  | (9.6 | ) |  | (9.3 | ) |  | (7.1 | ) |  | (6.6 | ) |  | (6.4 | )
Adjustment of prior period taxes | 0.8 |  |  | (1.3 | ) |  | (1.3 | ) |  | 0.7 |  |  | (3.0 | ) |  | (3.6 | ) |  | 0.1 |  |  | — |  |  | (0.2 | )
Other items, net | (1.1 | ) |  | (1.5 | ) |  | (1.8 | ) |  | (1.2 | ) |  | (1.2 | ) |  | (1.6 | ) |  | (0.8 | ) |  | (0.8 | ) |  | (0.8 | )
Overall income tax rate | 15.3 | % |  | 10.1 | % |  | 12.4 | % |  | (13.1 | %) |  | (34.2 | %) |  | (23.9 | %) |  | 31.8 | % |  | 32.0 | % |  | 32.3 | %
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 income tax expense and benefits are impacted by certain property-related differences at IPL for which deferred tax is not recognized in the income statement pursuant to Iowa rate-making principles. In 2014, the increased benefits from property-related differences were primarily due to additional repairs deductions and additional deductions from the allocation of mixed service costs related to Marshalltown. Production tax credits - Production tax credits are earned from owned and operated wind farms. Production tax credits are based on the electricity generated by each wind farm during the first 10 years of operation. Details regarding production tax credits (net of state tax impacts) related to various wind farms are as follows (dollars in millions):
| End of Production |  | Nameplate |  |  |  |  |  |
| Tax Credit Generation |  | Capacity in MW |  | 2015 |  | 2014 |  | 2013
Cedar Ridge (WPL) | December 2018 |  | 68 |  |  | $4.5 |  |  |  | $4.1 |  |  |  | $4.2 |
Bent Tree (WPL) | February 2021 |  | 201 |  | 14.1 |  |  | 13.4 |  |  | 12.7 |
Subtotal (WPL) |  |  |  |  | 18.6 |  |  | 17.5 |  |  | 16.9 |
Whispering Willow - East (IPL) | December 2019 |  | 200 |  | 14.5 |  |  | 13.8 |  |  | 14.1 |
|  |  |  |  |  | $33.1 |  |  |  | $31.3 |  |  |  | $31.0 |
Deferred Tax Assets and Liabilities - The deferred tax (assets) and liabilities included on the balance sheets at December 31 arise from the following temporary differences (in millions):
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
Alliant Energy | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $2,762.9 |  |  | $2,762.9 |  |  |  | $— |  |  | $2,627.8 |  |  | $2,627.8 |
Investment in ATC | — |  | 138.1 |  | 138.1 |  |  | — |  | 131.6 |  | 131.6 |
Net operating losses carryforwards - state | (38.3 | ) | — |  | (38.3 | ) |  | (45.7 | ) | — |  | (45.7 | )
Regulatory liability - IPL’s tax benefit riders | (66.1 | ) | — |  | (66.1 | ) |  | (100.9 | ) | — |  | (100.9 | )
Federal credit carryforwards | (236.4 | ) | — |  | (236.4 | ) |  | (201.0 | ) | — |  | (201.0 | )
Net operating losses carryforwards - federal | (250.9 | ) | — |  | (250.9 | ) |  | (332.8 | ) | — |  | (332.8 | )
Other | (85.4 | ) | 157.3 |  | 71.9 |  |  | (88.1 | ) | 180.1 |  | 92.0 |
|  | ($677.1 | ) |  | $3,058.3 |  |  | $2,381.2 |  |  |  | ($768.5 | ) |  | $2,939.5 |  |  | $2,171.0 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($150.1 | )
Non-current deferred tax liabilities |  | $2,381.2 |  |  | 2,321.1 |
Total net deferred tax liabilities |  | $2,381.2 |  |  |  | $2,171.0 |
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
IPL | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $1,587.8 |  |  | $1,587.8 |  |  |  | $— |  |  | $1,545.1 |  |  | $1,545.1 |
Regulatory liability - tax benefit riders | (66.1 | ) | — |  | (66.1 | ) |  | (100.9 | ) | — |  | (100.9 | )
Federal credit carryforwards | (81.7 | ) | — |  | (81.7 | ) |  | (66.5 | ) | — |  | (66.5 | )
Net operating losses carryforwards - federal | (113.1 | ) | — |  | (113.1 | ) |  | (157.8 | ) | — |  | (157.8 | )
Other | (36.7 | ) | 87.8 |  | 51.1 |  |  | (37.1 | ) | 83.5 |  | 46.4 |
|  | ($297.6 | ) |  | $1,675.6 |  |  | $1,378.0 |  |  |  | ($362.3 | ) |  | $1,628.6 |  |  | $1,266.3 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($104.0 | )
Non-current deferred tax liabilities |  | $1,378.0 |  |  | 1,370.3 |
Total net deferred tax liabilities |  | $1,378.0 |  |  |  | $1,266.3 |
| 2015 |  | 2014
| Deferred | Deferred Tax |  |  | Deferred | Deferred Tax |
WPL | Tax Assets | Liabilities | Net |  | Tax Assets | Liabilities | Net
Property |  | $— |  |  | $1,027.0 |  |  | $1,027.0 |  |  |  | $— |  |  | $963.0 |  |  | $963.0 |
Investment in ATC | — |  | 138.9 |  | 138.9 |  |  | — |  | 132.1 |  | 132.1 |
Federal credit carryforwards | (95.5 | ) | — |  | (95.5 | ) |  | (76.7 | ) | — |  | (76.7 | )
Net operating losses carryforwards - federal | (105.1 | ) | — |  | (105.1 | ) |  | (131.6 | ) | — |  | (131.6 | )
Other | (27.8 | ) | 67.9 |  | 40.1 |  |  | (37.1 | ) | 79.7 |  | 42.6 |
|  | ($228.4 | ) |  | $1,233.8 |  |  | $1,005.4 |  |  |  | ($245.4 | ) |  | $1,174.8 |  |  | $929.4 |
| 2015 |  | 2014
Current deferred tax assets | N/A |  |  | ($37.7 | )
Non-current deferred tax liabilities |  | $1,005.4 |  |  | 967.1 |
Total net deferred tax liabilities |  | $1,005.4 |  |  |  | $929.4 |
Property - Property-related differences were primarily related to accelerated depreciation, including bonus depreciation. In December 2015, the PATH Act was enacted. The most significant provisions of the PATH Act for Alliant Energy, IPL and WPL relate to the extension of bonus depreciation deductions for certain expenditures for property incurred through December 31, 2019 and placed in service prior to December 31, 2020. Alliant Energy currently estimates its total bonus depreciation deductions to be claimed on its U.S. federal income tax return for calendar year 2015 will be approximately $200 million ($100 million for IPL and $50 million for WPL). Investment in ATC - WPL Transco has a partial ownership interest in ATC, which has generated deferred tax liabilities primarily from tax depreciation deductions taken at ATC in excess of book depreciation. The increase in deferred tax liabilities in 2015 was primarily due to bonus depreciation deductions estimated at ATC. Regulatory liability tax benefit riders - Refer to Note 2 for discussion of regulatory liabilities associated with IPL’s tax benefit riders. Carryforwards - At December 31, 2015, tax carryforwards and associated deferred tax assets and expiration dates were estimated as follows (in millions):
|  |  | Alliant Energy |  | IPL |  | WPL
| Earliest |  | Tax |  | Deferred |  | Tax |  | Deferred |  | Tax |  | Deferred
| Expiration Date |  | Carryforwards |  | Tax Assets |  | Carryforwards |  | Tax Assets |  | Carryforwards |  | Tax Assets
Federal net operating losses | 2030 |  |  | $732 |  |  |  | $251 |  |  |  | $336 |  |  |  | $113 |  |  |  | $300 |  |  |  | $105 |
State net operating losses | 2018 |  | 766 |  |  | 38 |  |  | 20 |  |  | 1 |  |  | 76 |  |  | 4 |
Federal tax credits | 2022 |  | 240 |  |  | 236 |  |  | 85 |  |  | 82 |  |  | 96 |  |  | 96 |
|  |  |  |  |  | $525 |  |  |  |  |  | $196 |  |  |  |  |  | $205 |
At December 31, 2015, Alliant Energy’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 99% expiring after 2024. At December 31, 2015, IPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 95% expiring after 2021. At December 31, 2015, WPL’s state net operating losses carryforwards had expiration dates ranging from 2018 to 2034 with 96% expiring after 2033. Uncertain Tax Positions - At December 31, 2015, 2014 and 2013, there were no uncertain tax positions or penalties accrued related to uncertain tax positions, and interest accrued and tax positions favorably impacting future effective tax rates for continuing operations were not material. As of December 31, 2015, no material changes to unrecognized tax benefits are expected during the next 12 months. Open tax years - Tax years that remain subject to the statute of limitations in the major jurisdictions for each of Alliant Energy, IPL and WPL are as follows:
Consolidated federal income tax returns (a) | 2012 | - | 2014
Consolidated Iowa income tax returns (b) | 2012 | - | 2014
Wisconsin combined tax returns (c) | 2011 | - | 2014
(a) | These federal tax returns are effectively settled as a result of participation in the IRS Compliance Assurance Program, which allows Alliant Energy and the IRS to work together to resolve issues related to Alliant Energy’s current tax year before filing its federal income tax return. The statute of limitations for these federal tax returns expires three years from each filing date.
(b) | The statute of limitations for these Iowa tax returns expires three years from each filing date.
(c) | The statute of limitations for these Wisconsin combined tax returns expires four years from each filing date.

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