CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Other Comprehensive Income [Abstract]
Net income	$ 420.4	$ 390.4
Derivatives accounted for as cash flow hedges
Derivative losses, net of tax	(1.2)	0.0
Reclassification of net gains to net income, net of tax	(0.3)	(0.2)
Cash flow hedges, net	(1.5)	(0.2)
Defined benefit plans
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.1	1.9
Other comprehensive (loss) income, net of tax	(1.4)	1.7
Comprehensive income	419.0	392.1
Preferred stock dividends of subsidiary	0.3	0.3
Comprehensive income attributed to common shareholders	$ 418.7	$ 391.8