CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 356.9	$ 346.5
Derivatives accounted for as cash flow hedges
Reclassification of gains to net income, net of tax	(0.3)	(0.3)
Defined benefit plans
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.1	0.0
Other comprehensive loss, net of tax	(0.2)	(0.3)
Comprehensive income	356.7	346.2
Preferred stock dividends of subsidiary	0.3	0.3
Comprehensive income attributed to common shareholders	$ 356.4	$ 345.9