Common Equity - Stock-Based Compensation Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share Based Compensation Arrangement By Share Based Payment Award
Stock-based compensation expense	$ 9.7	$ 34.8	$ 15.7
Related tax benefit	2.7	9.6	4.3
Stock options
Share Based Compensation Arrangement By Share Based Payment Award
Stock-based compensation expense	5.3	6.5	6.5
Restricted stock
Share Based Compensation Arrangement By Share Based Payment Award
Stock-based compensation expense	6.6	7.0	6.1
Performance units
Share Based Compensation Arrangement By Share Based Payment Award
Stock-based compensation expense	$ (2.2)	$ 21.3	$ 3.1