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Common Equity - Stock-Based Compensation Expense (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Stock-based compensation expense | $ 50.2 | $ 36.1 | $ 29.0
Related tax benefit | 13.8 | 9.9 | 11.6
Stock options |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Stock-based compensation expense | 4.4 | 5.2 | 3.4
Restricted stock |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Stock-based compensation expense | 7.1 | 10.7 | 5.4
Performance units |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Stock-based compensation expense | $ 38.7 | $ 20.2 | $ 20.2

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