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Regulatory Matters (Tables) | 3 Months Ended
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Mar. 31, 2018
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017
Tax-related |  | $795.1 |  |  |  | $778.2 |  |  |  | $762.8 |  |  |  | $750.5 |  |  |  | $32.3 |  |  |  | $27.7 |
Pension and OPEB costs | 538.6 |  |  | 548.0 |  |  | 270.0 |  |  | 274.4 |  |  | 268.6 |  |  | 273.6 |
Asset retirement obligations | 104.3 |  |  | 109.3 |  |  | 67.9 |  |  | 72.5 |  |  | 36.4 |  |  | 36.8 |
EGUs retired early | 60.1 |  |  | 63.8 |  |  | 30.4 |  |  | 31.6 |  |  | 29.7 |  |  | 32.2 |
Derivatives | 47.6 |  |  | 45.3 |  |  | 25.5 |  |  | 21.8 |  |  | 22.1 |  |  | 23.5 |
Emission allowances | 25.4 |  |  | 25.5 |  |  | 25.4 |  |  | 25.5 |  |  | — |  |  | — |
Other | 93.0 |  |  | 96.6 |  |  | 48.9 |  |  | 55.3 |  |  | 44.1 |  |  | 41.3 |
|  | $1,664.1 |  |  |  | $1,666.7 |  |  |  | $1,230.9 |  |  |  | $1,231.6 |  |  |  | $433.2 |  |  |  | $435.1 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017
Tax-related |  | $901.3 |  |  |  | $899.4 |  |  |  | $399.5 |  |  |  | $399.5 |  |  |  | $501.8 |  |  |  | $499.9 |
Cost of removal obligations | 415.6 |  |  | 410.0 |  |  | 274.7 |  |  | 274.5 |  |  | 140.9 |  |  | 135.5 |
Electric transmission cost recovery | 95.3 |  |  | 90.4 |  |  | 33.5 |  |  | 26.4 |  |  | 61.8 |  |  | 64.0 |
Commodity cost recovery | 31.2 |  |  | 21.0 |  |  | 24.2 |  |  | 14.6 |  |  | 7.0 |  |  | 6.4 |
IPL’s tax benefit riders | 25.0 |  |  | 25.0 |  |  | 25.0 |  |  | 25.0 |  |  | — |  |  | — |
Other | 69.5 |  |  | 51.4 |  |  | 20.4 |  |  | 15.4 |  |  | 49.1 |  |  | 36.0 |
|  | $1,537.9 |  |  |  | $1,497.2 |  |  |  | $777.3 |  |  |  | $755.4 |  |  |  | $760.6 |  |  |  | $741.8 |
IPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017
Tax-related |  | $795.1 |  |  |  | $778.2 |  |  |  | $762.8 |  |  |  | $750.5 |  |  |  | $32.3 |  |  |  | $27.7 |
Pension and OPEB costs | 538.6 |  |  | 548.0 |  |  | 270.0 |  |  | 274.4 |  |  | 268.6 |  |  | 273.6 |
Asset retirement obligations | 104.3 |  |  | 109.3 |  |  | 67.9 |  |  | 72.5 |  |  | 36.4 |  |  | 36.8 |
EGUs retired early | 60.1 |  |  | 63.8 |  |  | 30.4 |  |  | 31.6 |  |  | 29.7 |  |  | 32.2 |
Derivatives | 47.6 |  |  | 45.3 |  |  | 25.5 |  |  | 21.8 |  |  | 22.1 |  |  | 23.5 |
Emission allowances | 25.4 |  |  | 25.5 |  |  | 25.4 |  |  | 25.5 |  |  | — |  |  | — |
Other | 93.0 |  |  | 96.6 |  |  | 48.9 |  |  | 55.3 |  |  | 44.1 |  |  | 41.3 |
|  | $1,664.1 |  |  |  | $1,666.7 |  |  |  | $1,230.9 |  |  |  | $1,231.6 |  |  |  | $433.2 |  |  |  | $435.1 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017
Tax-related |  | $901.3 |  |  |  | $899.4 |  |  |  | $399.5 |  |  |  | $399.5 |  |  |  | $501.8 |  |  |  | $499.9 |
Cost of removal obligations | 415.6 |  |  | 410.0 |  |  | 274.7 |  |  | 274.5 |  |  | 140.9 |  |  | 135.5 |
Electric transmission cost recovery | 95.3 |  |  | 90.4 |  |  | 33.5 |  |  | 26.4 |  |  | 61.8 |  |  | 64.0 |
Commodity cost recovery | 31.2 |  |  | 21.0 |  |  | 24.2 |  |  | 14.6 |  |  | 7.0 |  |  | 6.4 |
IPL’s tax benefit riders | 25.0 |  |  | 25.0 |  |  | 25.0 |  |  | 25.0 |  |  | — |  |  | — |
Other | 69.5 |  |  | 51.4 |  |  | 20.4 |  |  | 15.4 |  |  | 49.1 |  |  | 36.0 |
|  | $1,537.9 |  |  |  | $1,497.2 |  |  |  | $777.3 |  |  |  | $755.4 |  |  |  | $760.6 |  |  |  | $741.8 |
WPL [Member] |
Regulatory Assets [Line Items] |
Regulatory Assets | Regulatory assets were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017
Tax-related |  | $795.1 |  |  |  | $778.2 |  |  |  | $762.8 |  |  |  | $750.5 |  |  |  | $32.3 |  |  |  | $27.7 |
Pension and OPEB costs | 538.6 |  |  | 548.0 |  |  | 270.0 |  |  | 274.4 |  |  | 268.6 |  |  | 273.6 |
Asset retirement obligations | 104.3 |  |  | 109.3 |  |  | 67.9 |  |  | 72.5 |  |  | 36.4 |  |  | 36.8 |
EGUs retired early | 60.1 |  |  | 63.8 |  |  | 30.4 |  |  | 31.6 |  |  | 29.7 |  |  | 32.2 |
Derivatives | 47.6 |  |  | 45.3 |  |  | 25.5 |  |  | 21.8 |  |  | 22.1 |  |  | 23.5 |
Emission allowances | 25.4 |  |  | 25.5 |  |  | 25.4 |  |  | 25.5 |  |  | — |  |  | — |
Other | 93.0 |  |  | 96.6 |  |  | 48.9 |  |  | 55.3 |  |  | 44.1 |  |  | 41.3 |
|  | $1,664.1 |  |  |  | $1,666.7 |  |  |  | $1,230.9 |  |  |  | $1,231.6 |  |  |  | $433.2 |  |  |  | $435.1 |
Regulatory Liabilities | Regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017 |  | March 31, 2018 |  | December 31, 2017
Tax-related |  | $901.3 |  |  |  | $899.4 |  |  |  | $399.5 |  |  |  | $399.5 |  |  |  | $501.8 |  |  |  | $499.9 |
Cost of removal obligations | 415.6 |  |  | 410.0 |  |  | 274.7 |  |  | 274.5 |  |  | 140.9 |  |  | 135.5 |
Electric transmission cost recovery | 95.3 |  |  | 90.4 |  |  | 33.5 |  |  | 26.4 |  |  | 61.8 |  |  | 64.0 |
Commodity cost recovery | 31.2 |  |  | 21.0 |  |  | 24.2 |  |  | 14.6 |  |  | 7.0 |  |  | 6.4 |
IPL’s tax benefit riders | 25.0 |  |  | 25.0 |  |  | 25.0 |  |  | 25.0 |  |  | — |  |  | — |
Other | 69.5 |  |  | 51.4 |  |  | 20.4 |  |  | 15.4 |  |  | 49.1 |  |  | 36.0 |
|  | $1,537.9 |  |  |  | $1,497.2 |  |  |  | $777.3 |  |  |  | $755.4 |  |  |  | $760.6 |  |  |  | $741.8 |

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