CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Other Comprehensive Income [Abstract]
Net income (loss)	$ 510.3	$ 452.6
Derivatives accounted for as cash flow hedges
Net derivative loss, net of tax benefit of $—, and $1.3, respectively	0.0	(3.4)
Reclassification of realized net derivative loss to net income, net of tax	1.0	0.1
Cash flow hedges, net	1.0	(3.3)
Defined benefit plans
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.1	0.3
Other comprehensive income (loss), net of tax	1.1	(3.0)
Comprehensive income	511.4	449.6
Preferred stock dividends of subsidiary	0.3	0.3
Comprehensive loss attributed to noncontrolling interests	0.1	0.2
Comprehensive income attributed to common shareholders	$ 511.2	$ 449.5