CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Other Comprehensive Income [Abstract]
Net income	$ 288.7	$ 267.1	$ 1,074.7	$ 961.8
Derivatives accounted for as cash flow hedges
Net derivative loss, net of tax benefit of $— , $—, $—, and $(1.6), respectively	0.0	0.0	0.0	(4.2)
Reclassification of realized net derivative loss to net income, net of tax expense of $0.5 , $0.4, $1.2, and $0.6, respectively	0.9	0.9	2.9	1.4
Cash flow hedges, net	0.9	0.9	2.9	(2.8)
Defined benefit plans
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.1	0.0	0.3	0.5
Other comprehensive income (loss), net of tax	1.0	0.9	3.2	(2.3)
Comprehensive income	289.7	268.0	1,077.9	959.5
Preferred stock dividends of subsidiary	0.3	0.3	0.9	0.9
Comprehensive loss attributed to noncontrolling interests	1.6	0.0	2.3	0.0
Comprehensive income attributed to common shareholders	$ 291.0	$ 267.7	$ 1,079.3	$ 958.6