Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 940.2	$ 640.3	$ 589.5
Derivatives accounted for as cash flow hedges
Gains on settlement, net of tax of $7.6	0.0	11.4	0.0
Reclassification of gains to net income, net of tax	(1.3)	(0.8)	0.0
Cash flow hedges, net	(1.3)	10.6	0.0
Defined benefit plans
Pension and OPEB costs arising during the period, net of tax of $0.1 and $(4.2), respectively	(0.8)	(6.3)	0.0
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.4	0.0	0.0
Defined benefit plans, net	(0.4)	(6.3)	0.0
Other comprehensive (loss) income, net of tax	(1.7)	4.3	0.0
Comprehensive income	938.5	644.6	589.5
Preferred stock dividends of subsidiary	1.2	1.8	1.2
Comprehensive income attributed to common shareholders	$ 937.3	$ 642.8	$ 588.3