CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Other Comprehensive Income [Abstract]
Net income	$ 390.4	$ 356.9
Derivatives accounted for as cash flow hedges
Reclassification of gains to net income, net of tax	(0.2)	(0.3)
Defined benefit plans
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	1.9	0.1
Other comprehensive income (loss), net of tax	1.7	(0.2)
Comprehensive income	392.1	356.7
Preferred stock dividends of subsidiary	0.3	0.3
Comprehensive income attributed to common shareholders	$ 391.8	$ 356.4