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Regulatory Matters (Tables) | 9 Months Ended
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Sep. 30, 2018
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017
Tax-related |  | $798.3 |  |  |  | $778.2 |  |  |  | $762.4 |  |  |  | $750.5 |  |  |  | $35.9 |  |  |  | $27.7 |
Pension and OPEB costs | 519.8 |  |  | 548.0 |  |  | 261.2 |  |  | 274.4 |  |  | 258.6 |  |  | 273.6 |
Asset retirement obligations | 111.6 |  |  | 109.3 |  |  | 77.3 |  |  | 72.5 |  |  | 34.3 |  |  | 36.8 |
EGUs retired early | 110.4 |  |  | 63.8 |  |  | 57.2 |  |  | 31.6 |  |  | 53.2 |  |  | 32.2 |
Derivatives | 26.3 |  |  | 45.3 |  |  | 13.3 |  |  | 21.8 |  |  | 13.0 |  |  | 23.5 |
Emission allowances | 24.3 |  |  | 25.5 |  |  | 24.3 |  |  | 25.5 |  |  | — |  |  | — |
Other | 104.0 |  |  | 96.6 |  |  | 54.2 |  |  | 55.3 |  |  | 49.8 |  |  | 41.3 |
|  | $1,694.7 |  |  |  | $1,666.7 |  |  |  | $1,249.9 |  |  |  | $1,231.6 |  |  |  | $444.8 |  |  |  | $435.1 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017
Tax-related |  | $891.1 |  |  |  | $899.4 |  |  |  | $389.9 |  |  |  | $399.5 |  |  |  | $501.2 |  |  |  | $499.9 |
Cost of removal obligations | 401.0 |  |  | 410.0 |  |  | 274.0 |  |  | 274.5 |  |  | 127.0 |  |  | 135.5 |
Electric transmission cost recovery | 97.0 |  |  | 90.4 |  |  | 37.9 |  |  | 26.4 |  |  | 59.1 |  |  | 64.0 |
Commodity cost recovery | 20.1 |  |  | 21.0 |  |  | 13.6 |  |  | 14.6 |  |  | 6.5 |  |  | 6.4 |
IPL’s tax benefit riders | 13.0 |  |  | 25.0 |  |  | 13.0 |  |  | 25.0 |  |  | — |  |  | — |
Other | 83.4 |  |  | 51.4 |  |  | 34.2 |  |  | 15.4 |  |  | 49.2 |  |  | 36.0 |
|  | $1,505.6 |  |  |  | $1,497.2 |  |  |  | $762.6 |  |  |  | $755.4 |  |  |  | $743.0 |  |  |  | $741.8 |
IPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017
Tax-related |  | $798.3 |  |  |  | $778.2 |  |  |  | $762.4 |  |  |  | $750.5 |  |  |  | $35.9 |  |  |  | $27.7 |
Pension and OPEB costs | 519.8 |  |  | 548.0 |  |  | 261.2 |  |  | 274.4 |  |  | 258.6 |  |  | 273.6 |
Asset retirement obligations | 111.6 |  |  | 109.3 |  |  | 77.3 |  |  | 72.5 |  |  | 34.3 |  |  | 36.8 |
EGUs retired early | 110.4 |  |  | 63.8 |  |  | 57.2 |  |  | 31.6 |  |  | 53.2 |  |  | 32.2 |
Derivatives | 26.3 |  |  | 45.3 |  |  | 13.3 |  |  | 21.8 |  |  | 13.0 |  |  | 23.5 |
Emission allowances | 24.3 |  |  | 25.5 |  |  | 24.3 |  |  | 25.5 |  |  | — |  |  | — |
Other | 104.0 |  |  | 96.6 |  |  | 54.2 |  |  | 55.3 |  |  | 49.8 |  |  | 41.3 |
|  | $1,694.7 |  |  |  | $1,666.7 |  |  |  | $1,249.9 |  |  |  | $1,231.6 |  |  |  | $444.8 |  |  |  | $435.1 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017
Tax-related |  | $891.1 |  |  |  | $899.4 |  |  |  | $389.9 |  |  |  | $399.5 |  |  |  | $501.2 |  |  |  | $499.9 |
Cost of removal obligations | 401.0 |  |  | 410.0 |  |  | 274.0 |  |  | 274.5 |  |  | 127.0 |  |  | 135.5 |
Electric transmission cost recovery | 97.0 |  |  | 90.4 |  |  | 37.9 |  |  | 26.4 |  |  | 59.1 |  |  | 64.0 |
Commodity cost recovery | 20.1 |  |  | 21.0 |  |  | 13.6 |  |  | 14.6 |  |  | 6.5 |  |  | 6.4 |
IPL’s tax benefit riders | 13.0 |  |  | 25.0 |  |  | 13.0 |  |  | 25.0 |  |  | — |  |  | — |
Other | 83.4 |  |  | 51.4 |  |  | 34.2 |  |  | 15.4 |  |  | 49.2 |  |  | 36.0 |
|  | $1,505.6 |  |  |  | $1,497.2 |  |  |  | $762.6 |  |  |  | $755.4 |  |  |  | $743.0 |  |  |  | $741.8 |
WPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017
Tax-related |  | $798.3 |  |  |  | $778.2 |  |  |  | $762.4 |  |  |  | $750.5 |  |  |  | $35.9 |  |  |  | $27.7 |
Pension and OPEB costs | 519.8 |  |  | 548.0 |  |  | 261.2 |  |  | 274.4 |  |  | 258.6 |  |  | 273.6 |
Asset retirement obligations | 111.6 |  |  | 109.3 |  |  | 77.3 |  |  | 72.5 |  |  | 34.3 |  |  | 36.8 |
EGUs retired early | 110.4 |  |  | 63.8 |  |  | 57.2 |  |  | 31.6 |  |  | 53.2 |  |  | 32.2 |
Derivatives | 26.3 |  |  | 45.3 |  |  | 13.3 |  |  | 21.8 |  |  | 13.0 |  |  | 23.5 |
Emission allowances | 24.3 |  |  | 25.5 |  |  | 24.3 |  |  | 25.5 |  |  | — |  |  | — |
Other | 104.0 |  |  | 96.6 |  |  | 54.2 |  |  | 55.3 |  |  | 49.8 |  |  | 41.3 |
|  | $1,694.7 |  |  |  | $1,666.7 |  |  |  | $1,249.9 |  |  |  | $1,231.6 |  |  |  | $444.8 |  |  |  | $435.1 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017 |  | September 30, 2018 |  | December 31, 2017
Tax-related |  | $891.1 |  |  |  | $899.4 |  |  |  | $389.9 |  |  |  | $399.5 |  |  |  | $501.2 |  |  |  | $499.9 |
Cost of removal obligations | 401.0 |  |  | 410.0 |  |  | 274.0 |  |  | 274.5 |  |  | 127.0 |  |  | 135.5 |
Electric transmission cost recovery | 97.0 |  |  | 90.4 |  |  | 37.9 |  |  | 26.4 |  |  | 59.1 |  |  | 64.0 |
Commodity cost recovery | 20.1 |  |  | 21.0 |  |  | 13.6 |  |  | 14.6 |  |  | 6.5 |  |  | 6.4 |
IPL’s tax benefit riders | 13.0 |  |  | 25.0 |  |  | 13.0 |  |  | 25.0 |  |  | — |  |  | — |
Other | 83.4 |  |  | 51.4 |  |  | 34.2 |  |  | 15.4 |  |  | 49.2 |  |  | 36.0 |
|  | $1,505.6 |  |  |  | $1,497.2 |  |  |  | $762.6 |  |  |  | $755.4 |  |  |  | $743.0 |  |  |  | $741.8 |

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