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Summary Of Significant Accounting Policies (Tables)
 12 Months Ended
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.