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Common Equity - Stock-Based Compensation Expense (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense $ 36.1 $ 29.0 $ 18.0
Related tax benefit 9.9 11.6 7.2
Stock options      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense 5.2 3.4 3.5
Restricted stock      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense 10.7 5.4 5.8
Performance units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Stock-based compensation expense $ 20.2 $ 20.2 $ 8.7