CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Other Comprehensive Income [Abstract]
Net income	$ 231.3	$ 199.4	$ 621.7	$ 556.3
Derivatives accounted for as cash flow hedges
Reclassification of gains to net income, net of tax	(0.4)	(0.3)	(0.6)	(0.6)
Defined benefit plans
Amortization of pension and OPEB (credits) costs included in net periodic benefit cost, net of tax	(1.7)	0.1	0.2	0.2
Other comprehensive loss, net of tax	(2.1)	(0.2)	(0.4)	(0.4)
Comprehensive income	229.2	199.2	621.3	555.9
Preferred stock dividends of subsidiary	0.3	0.3	0.6	0.6
Comprehensive income attributed to common shareholders	$ 228.9	$ 198.9	$ 620.7	$ 555.3