CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Other Comprehensive Income [Abstract]
Net income	$ 234.3	$ 233.5	$ 890.5	$ 855.2
Derivatives accounted for as cash flow hedges
Derivative (losses) gains, net of tax of $(0.2), $0.1, $(1.5), and $0.1, respectively	(0.3)	0.3	(3.8)	0.3
Reclassification of net gains to net income, net of tax	(0.2)	(0.4)	(0.8)	(1.0)
Cash flow hedges, net	(0.5)	(0.1)	(4.6)	(0.7)
Defined benefit plans
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.0	0.0	0.1	0.2
Other comprehensive loss, net of tax	(0.5)	(0.1)	(4.5)	(0.5)
Comprehensive income	233.8	233.4	886.0	854.7
Preferred stock dividends of subsidiary	0.3	0.3	0.9	0.9
Comprehensive loss attributed to noncontrolling interests	0.3	0.0	0.5	0.0
Comprehensive income attributed to common shareholders	$ 233.8	$ 233.1	$ 885.6	$ 853.8