Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 181.7	$ 81.2	$ 528.2	$ 277.3
Derivatives accounted for as cash flow hedges
Gains on settlement, net of tax of $7.6	0.0	11.4	0.0	11.4
Reclassification of gains to net income, net of tax	(0.3)	(0.1)	(0.6)	(0.1)
Cash flow hedges, net	(0.3)	11.3	(0.6)	11.3
Defined benefit plans
Amortization of pension and OPEB costs included in net periodic benefit cost, net of tax	0.4	0.0	0.4	0.0
Other comprehensive income (loss), net of tax	0.1	11.3	(0.2)	11.3
Comprehensive income	181.8	92.5	528.0	288.6
Preferred stock dividends of subsidiary	0.3	0.3	0.6	0.6
Comprehensive income attributed to common shareholders	$ 181.5	$ 92.2	$ 527.4	$ 288.0