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Summary Of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2017
Schedule of Average Rates of Depreciation | The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows:
| IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Electric - generation | 3.5% |  | 3.5% |  | 3.6% |  | 3.5% |  | 3.1% |  | 3.2%
Electric - distribution | 2.4% |  | 2.4% |  | 2.4% |  | 2.6% |  | 2.6% |  | 2.7%
Electric - other | 4.5% |  | 4.2% |  | 4.0% |  | 6.9% |  | 4.7% |  | 4.5%
Gas | 3.4% |  | 3.3% |  | 3.2% |  | 2.5% |  | 2.5% |  | 2.5%
Other | 4.0% |  | 3.9% |  | 3.9% |  | 6.0% |  | 5.9% |  | 6.0%
Schedule of Allowance for Funds Used During Construction Rate | The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows:
| 2017 |  | 2016 |  | 2015
IPL (Marshalltown CWIP) (a) | 7.8% |  | 7.9% |  | 7.9%
IPL (Wind generation CWIP) (b) | 7.6% |  | N/A |  | N/A
IPL (other CWIP) | 7.6% |  | 7.7% |  | 7.7%
WPL (retail jurisdiction) | 7.6% |  | 8.2% |  | 8.2%
WPL (wholesale jurisdiction) | 6.0% |  | 6.7% |  | 7.9%
(a) | In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown.
(b) | In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding.
IPL [Member] |
Schedule of Average Rates of Depreciation | The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows:
| IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Electric - generation | 3.5% |  | 3.5% |  | 3.6% |  | 3.5% |  | 3.1% |  | 3.2%
Electric - distribution | 2.4% |  | 2.4% |  | 2.4% |  | 2.6% |  | 2.6% |  | 2.7%
Electric - other | 4.5% |  | 4.2% |  | 4.0% |  | 6.9% |  | 4.7% |  | 4.5%
Gas | 3.4% |  | 3.3% |  | 3.2% |  | 2.5% |  | 2.5% |  | 2.5%
Other | 4.0% |  | 3.9% |  | 3.9% |  | 6.0% |  | 5.9% |  | 6.0%
Schedule of Allowance for Funds Used During Construction Rate | The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows:
| 2017 |  | 2016 |  | 2015
IPL (Marshalltown CWIP) (a) | 7.8% |  | 7.9% |  | 7.9%
IPL (Wind generation CWIP) (b) | 7.6% |  | N/A |  | N/A
IPL (other CWIP) | 7.6% |  | 7.7% |  | 7.7%
WPL (retail jurisdiction) | 7.6% |  | 8.2% |  | 8.2%
WPL (wholesale jurisdiction) | 6.0% |  | 6.7% |  | 7.9%
(a) | In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown.
(b) | In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding.
WPL [Member] |
Schedule of Average Rates of Depreciation | The average rates of depreciation for electric, gas and other properties, consistent with current rate-making practices, were as follows:
| IPL |  | WPL
| 2017 |  | 2016 |  | 2015 |  | 2017 |  | 2016 |  | 2015
Electric - generation | 3.5% |  | 3.5% |  | 3.6% |  | 3.5% |  | 3.1% |  | 3.2%
Electric - distribution | 2.4% |  | 2.4% |  | 2.4% |  | 2.6% |  | 2.6% |  | 2.7%
Electric - other | 4.5% |  | 4.2% |  | 4.0% |  | 6.9% |  | 4.7% |  | 4.5%
Gas | 3.4% |  | 3.3% |  | 3.2% |  | 2.5% |  | 2.5% |  | 2.5%
Other | 4.0% |  | 3.9% |  | 3.9% |  | 6.0% |  | 5.9% |  | 6.0%
Schedule of Allowance for Funds Used During Construction Rate | The AFUDC rates, computed in accordance with the prescribed regulatory formula, were as follows:
| 2017 |  | 2016 |  | 2015
IPL (Marshalltown CWIP) (a) | 7.8% |  | 7.9% |  | 7.9%
IPL (Wind generation CWIP) (b) | 7.6% |  | N/A |  | N/A
IPL (other CWIP) | 7.6% |  | 7.7% |  | 7.7%
WPL (retail jurisdiction) | 7.6% |  | 8.2% |  | 8.2%
WPL (wholesale jurisdiction) | 6.0% |  | 6.7% |  | 7.9%
(a) | In 2013, the IUB issued an order establishing rate-making principles that require a 10.3% return on common equity for the calculation of AFUDC related to the construction of Marshalltown.
(b) | In 2016, the IUB issued an order establishing rate-making principles that require a return on common equity for the calculation of AFUDC related to the construction of up to 500 MW of new wind generation equal to the greater of 10.0% or whatever percentage the IUB finds reasonable during IPL’s most recent retail electric rate proceeding.

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