CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Other Comprehensive Income [Abstract]
Net income	$ 242.1	$ 235.8	$ 694.7	$ 656.2
Derivatives accounted for as cash flow hedges
Net derivative losses, net of tax benefits of $(0.3), $(0.9), $(1.6), and $(1.3), respectively	(0.8)	(2.3)	(4.2)	(3.5)
Reclassification of net loss (gains) to net income, net of tax	0.4	(0.3)	0.5	(0.6)
Cash flow hedges, net	(0.4)	(2.6)	(3.7)	(4.1)
Defined benefit plans
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.2	0.0	0.5	0.1
Other comprehensive loss	(0.2)	(2.6)	(3.2)	(4.0)
Comprehensive income	241.9	233.2	691.5	652.2
Preferred stock dividends of subsidiary	0.3	0.3	0.6	0.6
Comprehensive (income) loss attributed to noncontrolling interests	(0.2)	0.2	0.0	0.2
Comprehensive income attributed to common shareholders	$ 241.4	$ 233.1	$ 690.9	$ 651.8