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Regulatory Matters (Tables) | 12 Months Ended
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Dec. 31, 2017
Regulatory Assets [Line Items] |
Schedule of Regulatory Assets | At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $778.2 |  |  |  | $1,055.6 |  |  |  | $750.5 |  |  |  | $1,022.4 |  |  |  | $27.7 |  |  |  | $33.2 |
Pension and OPEB costs | 548.0 |  |  | 578.7 |  |  | 274.4 |  |  | 294.0 |  |  | 273.6 |  |  | 284.7 |
AROs | 109.3 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.8 |  |  | 41.6 |
EGUs retired early | 63.8 |  |  | 41.4 |  |  | 31.6 |  |  | — |  |  | 32.2 |  |  | 41.4 |
Derivatives | 45.3 |  |  | 30.7 |  |  | 21.8 |  |  | 10.0 |  |  | 23.5 |  |  | 20.7 |
Emission allowances | 25.5 |  |  | 26.2 |  |  | 25.5 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.6 |  |  | 76.6 |  |  | 55.3 |  |  | 41.9 |  |  | 41.3 |  |  | 34.7 |
|  | $1,666.7 |  |  |  | $1,915.1 |  |  |  | $1,231.6 |  |  |  | $1,458.8 |  |  |  | $435.1 |  |  |  | $456.3 |
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $899.4 |  |  |  | $7.7 |  |  |  | $399.5 |  |  |  | $1.9 |  |  |  | $499.9 |  |  |  | $5.8 |
Cost of removal obligations | 410.0 |  |  | 411.6 |  |  | 274.5 |  |  | 269.4 |  |  | 135.5 |  |  | 142.2 |
Electric transmission cost recovery | 90.4 |  |  | 72.0 |  |  | 26.4 |  |  | 35.7 |  |  | 64.0 |  |  | 36.3 |
IPL’s tax benefit riders | 25.0 |  |  | 83.5 |  |  | 25.0 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 21.0 |  |  | 30.8 |  |  | 14.6 |  |  | 17.8 |  |  | 6.4 |  |  | 13.0 |
Energy efficiency cost recovery | 19.9 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.9 |  |  | 20.5 |
Other | 31.5 |  |  | 54.9 |  |  | 15.4 |  |  | 22.5 |  |  | 16.1 |  |  | 32.4 |
|  | $1,497.2 |  |  |  | $681.0 |  |  |  | $755.4 |  |  |  | $430.8 |  |  |  | $741.8 |  |  |  | $250.2 |
Tax Benefit Riders | In 2017, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions):
Electric tax benefit rider credits |  | ($65 | )
Gas tax benefit rider credits | (6 | )
Rate-making accounting change for capitalized interest | 17 |
Tax Reform adjustment (Refer to Note 11) | (5 | )
|  | ($59 | )
Electric Tax Benefit Rider | Details for IPL’s electric tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) |  | $65 |  |  |  | $64 |  |  |  | $72 |
Income tax benefit resulting from decreased taxable income caused by credits | 27 |  |  | 27 |  |  | 30 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 38 |  |  | 37 |  |  | 42 |
Revenue Requirement Adjustment | The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider as follows (in millions):
| 2016 |  | 2015
Revenue requirement adjustment |  | $14 |  |  |  | $14 |
Gas Tax Benefit Rider | Details for IPL’s gas tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) |  | $6 |  |  |  | $12 |  |  |  | $12 |
Income tax benefit resulting from decreased taxable income caused by credits | 3 |  |  | 5 |  |  | 5 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 3 |  |  | 7 |  |  | 7 |
Customer Billing Credits | The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate review through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions):
| 2016 |  | 2015 |  | 2014
Billing credits to reduce retail electric customers’ bills |  | $9 |  |  |  | $24 |  |  |  | $72 |
IPL [Member] |
Regulatory Assets [Line Items] |
Schedule of Regulatory Assets | At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $778.2 |  |  |  | $1,055.6 |  |  |  | $750.5 |  |  |  | $1,022.4 |  |  |  | $27.7 |  |  |  | $33.2 |
Pension and OPEB costs | 548.0 |  |  | 578.7 |  |  | 274.4 |  |  | 294.0 |  |  | 273.6 |  |  | 284.7 |
AROs | 109.3 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.8 |  |  | 41.6 |
EGUs retired early | 63.8 |  |  | 41.4 |  |  | 31.6 |  |  | — |  |  | 32.2 |  |  | 41.4 |
Derivatives | 45.3 |  |  | 30.7 |  |  | 21.8 |  |  | 10.0 |  |  | 23.5 |  |  | 20.7 |
Emission allowances | 25.5 |  |  | 26.2 |  |  | 25.5 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.6 |  |  | 76.6 |  |  | 55.3 |  |  | 41.9 |  |  | 41.3 |  |  | 34.7 |
|  | $1,666.7 |  |  |  | $1,915.1 |  |  |  | $1,231.6 |  |  |  | $1,458.8 |  |  |  | $435.1 |  |  |  | $456.3 |
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $899.4 |  |  |  | $7.7 |  |  |  | $399.5 |  |  |  | $1.9 |  |  |  | $499.9 |  |  |  | $5.8 |
Cost of removal obligations | 410.0 |  |  | 411.6 |  |  | 274.5 |  |  | 269.4 |  |  | 135.5 |  |  | 142.2 |
Electric transmission cost recovery | 90.4 |  |  | 72.0 |  |  | 26.4 |  |  | 35.7 |  |  | 64.0 |  |  | 36.3 |
IPL’s tax benefit riders | 25.0 |  |  | 83.5 |  |  | 25.0 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 21.0 |  |  | 30.8 |  |  | 14.6 |  |  | 17.8 |  |  | 6.4 |  |  | 13.0 |
Energy efficiency cost recovery | 19.9 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.9 |  |  | 20.5 |
Other | 31.5 |  |  | 54.9 |  |  | 15.4 |  |  | 22.5 |  |  | 16.1 |  |  | 32.4 |
|  | $1,497.2 |  |  |  | $681.0 |  |  |  | $755.4 |  |  |  | $430.8 |  |  |  | $741.8 |  |  |  | $250.2 |
Tax Benefit Riders | In 2017, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities increased (decreased) by ($59) million as follows (in millions):
Electric tax benefit rider credits |  | ($65 | )
Gas tax benefit rider credits | (6 | )
Rate-making accounting change for capitalized interest | 17 |
Tax Reform adjustment (Refer to Note 11) | (5 | )
|  | ($59 | )
Electric Tax Benefit Rider | Details for IPL’s electric tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail electric customers’ bills with reduction to electric revenues (based on customers’ KWh usage) |  | $65 |  |  |  | $64 |  |  |  | $72 |
Income tax benefit resulting from decreased taxable income caused by credits | 27 |  |  | 27 |  |  | 30 |
Income tax benefit representing tax benefits realized from electric tax benefit rider | 38 |  |  | 37 |  |  | 42 |
Revenue Requirement Adjustment | The revenue requirement adjustment resulted in increases to electric revenues in Alliant Energy’s and IPL’s income statements and was recognized through the energy adjustment clause as a reduction of the credits on IPL’s Iowa retail electric customers’ bills from the electric tax benefit rider as follows (in millions):
| 2016 |  | 2015
Revenue requirement adjustment |  | $14 |  |  |  | $14 |
Gas Tax Benefit Rider | Details for IPL’s gas tax benefit rider are as follows (in millions):
| 2017 |  | 2016 |  | 2015
Credit to IPL’s Iowa retail gas customers’ bills with reduction to gas revenues (based on a fixed amount per day) |  | $6 |  |  |  | $12 |  |  |  | $12 |
Income tax benefit resulting from decreased taxable income caused by credits | 3 |  |  | 5 |  |  | 5 |
Income tax benefit representing tax benefits realized from gas tax benefit rider | 3 |  |  | 7 |  |  | 7 |
Customer Billing Credits | The IUB approved a settlement agreement in 2014 related to rates charged to IPL’s Iowa retail electric customers. The settlement agreement extended IPL’s Iowa retail electric base rates authorized in its 2009 Test Year rate review through 2016 and provided targeted retail electric customer billing credits of $105 million in aggregate. IPL recorded such billing credits as follows (in millions):
| 2016 |  | 2015 |  | 2014
Billing credits to reduce retail electric customers’ bills |  | $9 |  |  |  | $24 |  |  |  | $72 |
WPL [Member] |
Regulatory Assets [Line Items] |
Schedule of Regulatory Assets | At December 31, regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $778.2 |  |  |  | $1,055.6 |  |  |  | $750.5 |  |  |  | $1,022.4 |  |  |  | $27.7 |  |  |  | $33.2 |
Pension and OPEB costs | 548.0 |  |  | 578.7 |  |  | 274.4 |  |  | 294.0 |  |  | 273.6 |  |  | 284.7 |
AROs | 109.3 |  |  | 105.9 |  |  | 72.5 |  |  | 64.3 |  |  | 36.8 |  |  | 41.6 |
EGUs retired early | 63.8 |  |  | 41.4 |  |  | 31.6 |  |  | — |  |  | 32.2 |  |  | 41.4 |
Derivatives | 45.3 |  |  | 30.7 |  |  | 21.8 |  |  | 10.0 |  |  | 23.5 |  |  | 20.7 |
Emission allowances | 25.5 |  |  | 26.2 |  |  | 25.5 |  |  | 26.2 |  |  | — |  |  | — |
Other | 96.6 |  |  | 76.6 |  |  | 55.3 |  |  | 41.9 |  |  | 41.3 |  |  | 34.7 |
|  | $1,666.7 |  |  |  | $1,915.1 |  |  |  | $1,231.6 |  |  |  | $1,458.8 |  |  |  | $435.1 |  |  |  | $456.3 |
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2017 |  | 2016 |  | 2017 |  | 2016 |  | 2017 |  | 2016
Tax-related |  | $899.4 |  |  |  | $7.7 |  |  |  | $399.5 |  |  |  | $1.9 |  |  |  | $499.9 |  |  |  | $5.8 |
Cost of removal obligations | 410.0 |  |  | 411.6 |  |  | 274.5 |  |  | 269.4 |  |  | 135.5 |  |  | 142.2 |
Electric transmission cost recovery | 90.4 |  |  | 72.0 |  |  | 26.4 |  |  | 35.7 |  |  | 64.0 |  |  | 36.3 |
IPL’s tax benefit riders | 25.0 |  |  | 83.5 |  |  | 25.0 |  |  | 83.5 |  |  | — |  |  | — |
Commodity cost recovery | 21.0 |  |  | 30.8 |  |  | 14.6 |  |  | 17.8 |  |  | 6.4 |  |  | 13.0 |
Energy efficiency cost recovery | 19.9 |  |  | 20.5 |  |  | — |  |  | — |  |  | 19.9 |  |  | 20.5 |
Other | 31.5 |  |  | 54.9 |  |  | 15.4 |  |  | 22.5 |  |  | 16.1 |  |  | 32.4 |
|  | $1,497.2 |  |  |  | $681.0 |  |  |  | $755.4 |  |  |  | $430.8 |  |  |  | $741.8 |  |  |  | $250.2 |

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