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Regulatory Matters (Tables) | 3 Months Ended
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Mar. 31, 2015
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014
Tax-related |  | $968.2 |  |  |  | $955.3 |  |  |  | $940.5 |  |  |  | $928.0 |  |  |  | $27.7 |  |  |  | $27.3 |
Pension and OPEB costs | 562.9 |  |  | 570.2 |  |  | 284.8 |  |  | 287.9 |  |  | 278.1 |  |  | 282.3 |
AROs | 75.6 |  |  | 73.7 |  |  | 42.5 |  |  | 41.4 |  |  | 33.1 |  |  | 32.3 |
Derivatives | 49.6 |  |  | 46.9 |  |  | 26.4 |  |  | 28.0 |  |  | 23.2 |  |  | 18.9 |
Commodity cost recovery | 32.1 |  |  | 31.1 |  |  | 0.6 |  |  | 0.4 |  |  | 31.5 |  |  | 30.7 |
Emission allowances | 26.9 |  |  | 27.4 |  |  | 26.9 |  |  | 27.4 |  |  | — |  |  | — |
Other | 75.0 |  |  | 79.1 |  |  | 42.0 |  |  | 44.8 |  |  | 33.0 |  |  | 34.3 |
|  | $1,790.3 |  |  |  | $1,783.7 |  |  |  | $1,363.7 |  |  |  | $1,357.9 |  |  |  | $426.6 |  |  |  | $425.8 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014
Cost of removal obligations |  | $423.2 |  |  |  | $421.7 |  |  |  | $279.5 |  |  |  | $279.1 |  |  |  | $143.7 |  |  |  | $142.6 |
IPL’s tax benefit riders | 222.0 |  |  | 243.0 |  |  | 222.0 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 62.2 |  |  | 64.3 |  |  | 1.6 |  |  | — |  |  | 60.6 |  |  | 64.3 |
Commodity cost recovery | 26.5 |  |  | 15.4 |  |  | 22.8 |  |  | 15.1 |  |  | 3.7 |  |  | 0.3 |
IPL’s electric transmission cost recovery | 23.2 |  |  | 19.4 |  |  | 23.2 |  |  | 19.4 |  |  | — |  |  | — |
Other | 59.0 |  |  | 57.4 |  |  | 23.0 |  |  | 26.9 |  |  | 36.0 |  |  | 30.5 |
|  | $816.1 |  |  |  | $821.2 |  |  |  | $572.1 |  |  |  | $583.5 |  |  |  | $244.0 |  |  |  | $237.7 |
Tax Benefit Riders | For the three months ended March 31, 2015, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $21 million as follows (in millions):
Electric tax benefit rider credits |  | $18 |
Gas tax benefit rider credits | 3 |
|  | $21 |
Customer Billing Credits | For the three months ended March 31, IPL recorded billing credits to reduce retail electric customers’ bills as follows (in millions):
| 2015
Billing credits to reduce retail electric customers’ bills | $6
IPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014
Tax-related |  | $968.2 |  |  |  | $955.3 |  |  |  | $940.5 |  |  |  | $928.0 |  |  |  | $27.7 |  |  |  | $27.3 |
Pension and OPEB costs | 562.9 |  |  | 570.2 |  |  | 284.8 |  |  | 287.9 |  |  | 278.1 |  |  | 282.3 |
AROs | 75.6 |  |  | 73.7 |  |  | 42.5 |  |  | 41.4 |  |  | 33.1 |  |  | 32.3 |
Derivatives | 49.6 |  |  | 46.9 |  |  | 26.4 |  |  | 28.0 |  |  | 23.2 |  |  | 18.9 |
Commodity cost recovery | 32.1 |  |  | 31.1 |  |  | 0.6 |  |  | 0.4 |  |  | 31.5 |  |  | 30.7 |
Emission allowances | 26.9 |  |  | 27.4 |  |  | 26.9 |  |  | 27.4 |  |  | — |  |  | — |
Other | 75.0 |  |  | 79.1 |  |  | 42.0 |  |  | 44.8 |  |  | 33.0 |  |  | 34.3 |
|  | $1,790.3 |  |  |  | $1,783.7 |  |  |  | $1,363.7 |  |  |  | $1,357.9 |  |  |  | $426.6 |  |  |  | $425.8 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014
Cost of removal obligations |  | $423.2 |  |  |  | $421.7 |  |  |  | $279.5 |  |  |  | $279.1 |  |  |  | $143.7 |  |  |  | $142.6 |
IPL’s tax benefit riders | 222.0 |  |  | 243.0 |  |  | 222.0 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 62.2 |  |  | 64.3 |  |  | 1.6 |  |  | — |  |  | 60.6 |  |  | 64.3 |
Commodity cost recovery | 26.5 |  |  | 15.4 |  |  | 22.8 |  |  | 15.1 |  |  | 3.7 |  |  | 0.3 |
IPL’s electric transmission cost recovery | 23.2 |  |  | 19.4 |  |  | 23.2 |  |  | 19.4 |  |  | — |  |  | — |
Other | 59.0 |  |  | 57.4 |  |  | 23.0 |  |  | 26.9 |  |  | 36.0 |  |  | 30.5 |
|  | $816.1 |  |  |  | $821.2 |  |  |  | $572.1 |  |  |  | $583.5 |  |  |  | $244.0 |  |  |  | $237.7 |
Tax Benefit Riders | For the three months ended March 31, 2015, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $21 million as follows (in millions):
Electric tax benefit rider credits |  | $18 |
Gas tax benefit rider credits | 3 |
|  | $21 |
Customer Billing Credits | For the three months ended March 31, IPL recorded billing credits to reduce retail electric customers’ bills as follows (in millions):
| 2015
Billing credits to reduce retail electric customers’ bills | $6
WPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014
Tax-related |  | $968.2 |  |  |  | $955.3 |  |  |  | $940.5 |  |  |  | $928.0 |  |  |  | $27.7 |  |  |  | $27.3 |
Pension and OPEB costs | 562.9 |  |  | 570.2 |  |  | 284.8 |  |  | 287.9 |  |  | 278.1 |  |  | 282.3 |
AROs | 75.6 |  |  | 73.7 |  |  | 42.5 |  |  | 41.4 |  |  | 33.1 |  |  | 32.3 |
Derivatives | 49.6 |  |  | 46.9 |  |  | 26.4 |  |  | 28.0 |  |  | 23.2 |  |  | 18.9 |
Commodity cost recovery | 32.1 |  |  | 31.1 |  |  | 0.6 |  |  | 0.4 |  |  | 31.5 |  |  | 30.7 |
Emission allowances | 26.9 |  |  | 27.4 |  |  | 26.9 |  |  | 27.4 |  |  | — |  |  | — |
Other | 75.0 |  |  | 79.1 |  |  | 42.0 |  |  | 44.8 |  |  | 33.0 |  |  | 34.3 |
|  | $1,790.3 |  |  |  | $1,783.7 |  |  |  | $1,363.7 |  |  |  | $1,357.9 |  |  |  | $426.6 |  |  |  | $425.8 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014 |  | March 31, 2015 |  | December 31, 2014
Cost of removal obligations |  | $423.2 |  |  |  | $421.7 |  |  |  | $279.5 |  |  |  | $279.1 |  |  |  | $143.7 |  |  |  | $142.6 |
IPL’s tax benefit riders | 222.0 |  |  | 243.0 |  |  | 222.0 |  |  | 243.0 |  |  | — |  |  | — |
Energy efficiency cost recovery | 62.2 |  |  | 64.3 |  |  | 1.6 |  |  | — |  |  | 60.6 |  |  | 64.3 |
Commodity cost recovery | 26.5 |  |  | 15.4 |  |  | 22.8 |  |  | 15.1 |  |  | 3.7 |  |  | 0.3 |
IPL’s electric transmission cost recovery | 23.2 |  |  | 19.4 |  |  | 23.2 |  |  | 19.4 |  |  | — |  |  | — |
Other | 59.0 |  |  | 57.4 |  |  | 23.0 |  |  | 26.9 |  |  | 36.0 |  |  | 30.5 |
|  | $816.1 |  |  |  | $821.2 |  |  |  | $572.1 |  |  |  | $583.5 |  |  |  | $244.0 |  |  |  | $237.7 |

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