Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Other Comprehensive Income [Abstract]
Net income	$ 1,331.7	$ 1,409.7	$ 1,298.5
Derivatives accounted for as cash flow hedges
Net derivative gain, net of tax	0.0	0.0	0.6
Reclassification of realized net derivative (gain) loss to net income, net of tax	(0.3)	(0.3)	0.9
Cash flow hedges, net	(0.3)	(0.3)	1.5
Defined benefit plans
Pension and OPEB adjustments arising during the period, net of tax expense (benefit) of $(0.2), $(1.3), and $0.7, respectively	(0.6)	(3.5)	1.7
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.0	0.2	0.4
Defined benefit plans, net	(0.6)	(3.3)	2.1
Other comprehensive income (loss), net of tax	(0.9)	(3.6)	3.6
Comprehensive income	1,330.8	1,406.1	1,302.1
Preferred stock dividends of subsidiary	1.2	1.2	1.2
Comprehensive (income) loss attributed to noncontrolling interests	1.2	(0.4)	3.0
Comprehensive income attributed to common shareholders	$ 1,330.8	$ 1,404.5	$ 1,303.9