Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 2,076.8	$ 1,643.7	$ 1,672.5	$ 2,286.5	$ 2,055.0	$ 1,657.5	$ 1,631.5	$ 2,304.5	$ 7,679.5	$ 7,648.5	$ 7,472.3
Operating income	289.8	302.7	330.8	545.1	407.0	392.2	362.2	614.7	1,468.4	1,776.1	1,696.3
Net income attributed to common shareholders	$ 205.0	$ 233.2	$ 231.0	$ 390.1	$ 432.6	$ 215.4	$ 199.1	$ 356.6	$ 1,059.3	$ 1,203.7	$ 939.0
Earnings Per Share, Basic
Earnings per common share (basic) (in dollars per share)	$ 0.65	$ 0.74	$ 0.73	$ 1.24	$ 1.37	$ 0.68	$ 0.63	$ 1.13	$ 3.36	$ 3.81	$ 2.98
Earnings Per Share (Diluted)
Earnings per common share (diluted) (in dollars per share)	$ 0.65	$ 0.74	$ 0.73	$ 1.23	$ 1.36	$ 0.68	$ 0.63	$ 1.12	$ 3.34	$ 3.79	$ 2.96