Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 711.0	$ 691.2	$ 630.7	$ 624.5	$ 699.0	$ 631.0	$ 533.7	$ 584.8	$ 2,657.3	$ 2,448.3	$ 2,434.7
Operating income	36.8	107.5	72.8	101.4	120.7	105.5	63.9	92.2	318.4	381.5	361.8
Net income	$ 61.2	$ 61.9	$ 37.6	$ 55.4	$ 69.6	$ 61.4	$ 32.3	$ 48.3	$ 216.0	$ 211.6	$ 197.3
Earnings per share [Abstract]
Basic and Diluted earnings per share (in dollars per share)	$ 0.74	$ 0.75	$ 0.45	$ 0.67	$ 0.84	$ 0.74	$ 0.39	$ 0.58	$ 2.60	$ 2.55	$ 2.39