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Regulatory Matters (Tables) | 12 Months Ended
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Dec. 31, 2018
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
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $820.6 |  |  |  | $778.2 |  |  |  | $783.1 |  |  |  | $750.5 |  |  |  | $37.5 |  |  |  | $27.7 |
Pension and OPEB costs | 542.3 |  |  | 548.0 |  |  | 274.0 |  |  | 274.4 |  |  | 268.3 |  |  | 273.6 |
EGUs retired early | 111.6 |  |  | 63.8 |  |  | 55.4 |  |  | 31.6 |  |  | 56.2 |  |  | 32.2 |
AROs | 110.8 |  |  | 109.3 |  |  | 76.3 |  |  | 72.5 |  |  | 34.5 |  |  | 36.8 |
Derivatives | 28.0 |  |  | 45.3 |  |  | 15.1 |  |  | 21.8 |  |  | 12.9 |  |  | 23.5 |
Emission allowances | 23.6 |  |  | 25.5 |  |  | 23.6 |  |  | 25.5 |  |  | — |  |  | — |
Other | 100.4 |  |  | 96.6 |  |  | 51.5 |  |  | 55.3 |  |  | 48.9 |  |  | 41.3 |
|  | $1,737.3 |  |  |  | $1,666.7 |  |  |  | $1,279.0 |  |  |  | $1,231.6 |  |  |  | $458.3 |  |  |  | $435.1 |
Electric Generating Units Retired Early | Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:
Entity |  | EGU |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2018 |  | Recovery |  | Regulatory Approval
IPL |  | Sutherland Units 1 and 3 |  | June 2017 |  |  | $27.6 |  |  | Return of remaining net book value over 10 years |  | IUB and FERC
IPL |  | M.L. Kapp Unit 2 |  | June 2018 |  | 27.8 |  |  | Return of and return on remaining net book value over 10 years |  | Pending with FERC; to be addressed with IUB
WPL |  | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 |  | December 2015 |  | 27.4 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and FERC
WPL |  | Edgewater Unit 4 |  | September 2018 |  | 28.8 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and pending with FERC
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $890.6 |  |  |  | $899.4 |  |  |  | $390.1 |  |  |  | $399.5 |  |  |  | $500.5 |  |  |  | $499.9 |
Cost of removal obligations | 401.2 |  |  | 410.0 |  |  | 273.3 |  |  | 274.5 |  |  | 127.9 |  |  | 135.5 |
Electric transmission cost recovery | 104.0 |  |  | 90.4 |  |  | 47.7 |  |  | 26.4 |  |  | 56.3 |  |  | 64.0 |
WPL earnings sharing mechanism | 25.4 |  |  | 8.0 |  |  | — |  |  | — |  |  | 25.4 |  |  | 8.0 |
Commodity cost recovery | 16.8 |  |  | 21.0 |  |  | 11.9 |  |  | 14.6 |  |  | 4.9 |  |  | 6.4 |
IPL’s tax benefit riders | 6.4 |  |  | 25.0 |  |  | 6.4 |  |  | 25.0 |  |  | — |  |  | — |
Other | 48.8 |  |  | 43.4 |  |  | 25.5 |  |  | 15.4 |  |  | 23.3 |  |  | 28.0 |
|  | $1,493.2 |  |  |  | $1,497.2 |  |  |  | $754.9 |  |  |  | $755.4 |  |  |  | $738.3 |  |  |  | $741.8 |
Tax Benefit Riders | In 2018, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions):
Electric tax benefit rider credits |  | $17 |
Gas tax benefit rider credits | 2 |
|  | $19 |
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
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $820.6 |  |  |  | $778.2 |  |  |  | $783.1 |  |  |  | $750.5 |  |  |  | $37.5 |  |  |  | $27.7 |
Pension and OPEB costs | 542.3 |  |  | 548.0 |  |  | 274.0 |  |  | 274.4 |  |  | 268.3 |  |  | 273.6 |
EGUs retired early | 111.6 |  |  | 63.8 |  |  | 55.4 |  |  | 31.6 |  |  | 56.2 |  |  | 32.2 |
AROs | 110.8 |  |  | 109.3 |  |  | 76.3 |  |  | 72.5 |  |  | 34.5 |  |  | 36.8 |
Derivatives | 28.0 |  |  | 45.3 |  |  | 15.1 |  |  | 21.8 |  |  | 12.9 |  |  | 23.5 |
Emission allowances | 23.6 |  |  | 25.5 |  |  | 23.6 |  |  | 25.5 |  |  | — |  |  | — |
Other | 100.4 |  |  | 96.6 |  |  | 51.5 |  |  | 55.3 |  |  | 48.9 |  |  | 41.3 |
|  | $1,737.3 |  |  |  | $1,666.7 |  |  |  | $1,279.0 |  |  |  | $1,231.6 |  |  |  | $458.3 |  |  |  | $435.1 |
Electric Generating Units Retired Early | Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:
Entity |  | EGU |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2018 |  | Recovery |  | Regulatory Approval
IPL |  | Sutherland Units 1 and 3 |  | June 2017 |  |  | $27.6 |  |  | Return of remaining net book value over 10 years |  | IUB and FERC
IPL |  | M.L. Kapp Unit 2 |  | June 2018 |  | 27.8 |  |  | Return of and return on remaining net book value over 10 years |  | Pending with FERC; to be addressed with IUB
WPL |  | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 |  | December 2015 |  | 27.4 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and FERC
WPL |  | Edgewater Unit 4 |  | September 2018 |  | 28.8 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and pending with FERC
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $890.6 |  |  |  | $899.4 |  |  |  | $390.1 |  |  |  | $399.5 |  |  |  | $500.5 |  |  |  | $499.9 |
Cost of removal obligations | 401.2 |  |  | 410.0 |  |  | 273.3 |  |  | 274.5 |  |  | 127.9 |  |  | 135.5 |
Electric transmission cost recovery | 104.0 |  |  | 90.4 |  |  | 47.7 |  |  | 26.4 |  |  | 56.3 |  |  | 64.0 |
WPL earnings sharing mechanism | 25.4 |  |  | 8.0 |  |  | — |  |  | — |  |  | 25.4 |  |  | 8.0 |
Commodity cost recovery | 16.8 |  |  | 21.0 |  |  | 11.9 |  |  | 14.6 |  |  | 4.9 |  |  | 6.4 |
IPL’s tax benefit riders | 6.4 |  |  | 25.0 |  |  | 6.4 |  |  | 25.0 |  |  | — |  |  | — |
Other | 48.8 |  |  | 43.4 |  |  | 25.5 |  |  | 15.4 |  |  | 23.3 |  |  | 28.0 |
|  | $1,493.2 |  |  |  | $1,497.2 |  |  |  | $754.9 |  |  |  | $755.4 |  |  |  | $738.3 |  |  |  | $741.8 |
Tax Benefit Riders | In 2018, Alliant Energy’s and IPL’s “IPL’s tax benefit riders” regulatory liabilities decreased by $19 million as follows (in millions):
Electric tax benefit rider credits |  | $17 |
Gas tax benefit rider credits | 2 |
|  | $19 |
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
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $820.6 |  |  |  | $778.2 |  |  |  | $783.1 |  |  |  | $750.5 |  |  |  | $37.5 |  |  |  | $27.7 |
Pension and OPEB costs | 542.3 |  |  | 548.0 |  |  | 274.0 |  |  | 274.4 |  |  | 268.3 |  |  | 273.6 |
EGUs retired early | 111.6 |  |  | 63.8 |  |  | 55.4 |  |  | 31.6 |  |  | 56.2 |  |  | 32.2 |
AROs | 110.8 |  |  | 109.3 |  |  | 76.3 |  |  | 72.5 |  |  | 34.5 |  |  | 36.8 |
Derivatives | 28.0 |  |  | 45.3 |  |  | 15.1 |  |  | 21.8 |  |  | 12.9 |  |  | 23.5 |
Emission allowances | 23.6 |  |  | 25.5 |  |  | 23.6 |  |  | 25.5 |  |  | — |  |  | — |
Other | 100.4 |  |  | 96.6 |  |  | 51.5 |  |  | 55.3 |  |  | 48.9 |  |  | 41.3 |
|  | $1,737.3 |  |  |  | $1,666.7 |  |  |  | $1,279.0 |  |  |  | $1,231.6 |  |  |  | $458.3 |  |  |  | $435.1 |
Electric Generating Units Retired Early | Details regarding the recovery of the remaining net book value of these EGUs from IPL’s and WPL’s customers are as follows:
Entity |  | EGU |  | Retirement Date |  | Regulatory Asset Balance as of Dec. 31, 2018 |  | Recovery |  | Regulatory Approval
IPL |  | Sutherland Units 1 and 3 |  | June 2017 |  |  | $27.6 |  |  | Return of remaining net book value over 10 years |  | IUB and FERC
IPL |  | M.L. Kapp Unit 2 |  | June 2018 |  | 27.8 |  |  | Return of and return on remaining net book value over 10 years |  | Pending with FERC; to be addressed with IUB
WPL |  | Nelson Dewey Units 1 and 2 and Edgewater Unit 3 |  | December 2015 |  | 27.4 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and FERC
WPL |  | Edgewater Unit 4 |  | September 2018 |  | 28.8 |  |  | Return of and return on remaining net book value over 10 years |  | PSCW and pending with FERC
Schedule of Regulatory Liabilities | At December 31, regulatory liabilities were comprised of the following items (in millions):
| Alliant Energy |  | IPL |  | WPL
| 2018 |  | 2017 |  | 2018 |  | 2017 |  | 2018 |  | 2017
Tax-related |  | $890.6 |  |  |  | $899.4 |  |  |  | $390.1 |  |  |  | $399.5 |  |  |  | $500.5 |  |  |  | $499.9 |
Cost of removal obligations | 401.2 |  |  | 410.0 |  |  | 273.3 |  |  | 274.5 |  |  | 127.9 |  |  | 135.5 |
Electric transmission cost recovery | 104.0 |  |  | 90.4 |  |  | 47.7 |  |  | 26.4 |  |  | 56.3 |  |  | 64.0 |
WPL earnings sharing mechanism | 25.4 |  |  | 8.0 |  |  | — |  |  | — |  |  | 25.4 |  |  | 8.0 |
Commodity cost recovery | 16.8 |  |  | 21.0 |  |  | 11.9 |  |  | 14.6 |  |  | 4.9 |  |  | 6.4 |
IPL’s tax benefit riders | 6.4 |  |  | 25.0 |  |  | 6.4 |  |  | 25.0 |  |  | — |  |  | — |
Other | 48.8 |  |  | 43.4 |  |  | 25.5 |  |  | 15.4 |  |  | 23.3 |  |  | 28.0 |
|  | $1,493.2 |  |  |  | $1,497.2 |  |  |  | $754.9 |  |  |  | $755.4 |  |  |  | $738.3 |  |  |  | $741.8 |

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