Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,156.3	$ 1,183.9	$ 1,212.7	$ 1,194.1	$ 1,321.6	$ 1,280.1	$ 1,236.3	$ 1,139.0		$ 4,747.0	$ 4,977.0	$ 4,383.6
Income from operations	78.1	29.0	49.2	51.5	118.6	97.9	91.7	67.6		207.8	375.8	280.3
Net income	$ 55.9	$ 19.7	$ 34.5	$ 36.9	$ 84.8	$ 70.7	$ 65.8	$ 47.6	$ 367.4	$ 147.0	$ 268.9	$ 389.9
Basic earnings per common share	$ 0.32	$ 0.11	$ 0.19	$ 0.21	$ 0.48	$ 0.40	$ 0.37	$ 0.27		$ 0.83	$ 1.52	$ 2.28
Diluted earnings per common share	$ 0.32	$ 0.11	$ 0.19	$ 0.21	$ 0.48	$ 0.40	$ 0.37	$ 0.27		$ 0.83	$ 1.52	$ 2.28