Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 182.5	$ 126.3	$ 459.2	$ 466.9
Derivatives accounted for as cash flow hedges
Gains on settlement, net of tax of $7.6 million	0.0	0.0	11.4	0.0
Reclassification of gains to net income, net of tax	(0.4)	0.0	(0.5)	0.0
Other comprehensive (loss) income, net of tax	(0.4)	0.0	10.9	0.0
Comprehensive income	$ 182.1	$ 126.3	$ 470.1	$ 466.9