Common Equity - Stock-Based Compensation Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share Based Compensation Arrangement By Share Based Payment Award
Stock-based compensation expense	$ 15.7	$ 35.7	$ 50.2
Related tax benefit	4.3	9.8	13.8
Stock options
Share Based Compensation Arrangement By Share Based Payment Award
Stock-based compensation expense	6.5	6.0	4.4
Restricted stock
Share Based Compensation Arrangement By Share Based Payment Award
Stock-based compensation expense	6.1	7.4	7.1
Performance units
Share Based Compensation Arrangement By Share Based Payment Award
Stock-based compensation expense	$ 3.1	$ 22.3	$ 38.7