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	$ 22.3	$ 14.6	$ 12.0
Related Tax Benefit	6.1	4.0	4.8
Stock options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1.7	2.0	1.3
Restricted stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	2.7	3.0	0.8
Performance units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 17.9	$ 9.6	$ 9.9