Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 217.3	$ 183.1	$ 745.5	$ 460.4
Derivatives accounted for as cash flow hedges
Gains on settlement, net of tax of $7.6	0.0	0.0	0.0	11.4
Reclassification of gains to net income, net of tax	(0.3)	(0.4)	(0.9)	(0.5)
Cash flow hedges, net	(0.3)	(0.4)	(0.9)	10.9
Defined benefit plans
Amortization of pension and OPEB credits included in net periodic benefit cost, net of tax	(0.4)	0.0	0.0	0.0
Other comprehensive (loss) income, net of tax	(0.7)	(0.4)	(0.9)	10.9
Comprehensive income	216.6	182.7	744.6	471.3
Preferred stock dividends of subsidiary	0.3	0.6	0.9	1.2
Comprehensive income attributed to common shareholders	$ 216.3	$ 182.1	$ 743.7	$ 470.1