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Regulatory Assets and Liabilities - Plant Retirements (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Regulatory assets |  |  |
Regulatory asset | $ 3,544.1 | $ 3,527.6 |
Deferred tax liabilities | 3,721.0 | 3,609.0 |
Severance liability for plant retirements |  |  |
Severance liability at January 1 | 2.1 | 15.7 | $ 29.4
Severance payments | (0.1) | (7.2) | (10.7)
Other | (1.3) | (6.4) | (3.0)
Severance liability at December 31 | 0.7 | 2.1 | $ 15.7
Securitization |  |  |
Regulatory assets |  |  |
Regulatory asset | 105.2 | $ 100.0 |
Edgewater Unit 4 |  |  |
Regulatory assets |  |  |
Regulatory asset | 4.7 |  |
Pleasant Prairie power plant |  |  |
Regulatory assets |  |  |
Net book value of retired plant | 602.7 |  |
Deferred unprotected tax benefits | 19.6 |  |
Regulatory asset | 583.1 |  |
Deferred tax liabilities | 168.7 |  |
Pleasant Prairie power plant | Securitization |  |  |
Regulatory assets |  |  |
Regulatory asset | 100.0 |  |
Presque Isle power plant |  |  |
Regulatory assets |  |  |
Net book value of retired plant | 161.0 |  |
Deferred unprotected tax benefits | 6.0 |  |
Regulatory asset | 155.0 |  |
Deferred tax liabilities | 46.0 |  |
Pulliam power plant |  |  |
Regulatory assets |  |  |
Regulatory asset | $ 42.6 |  |

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