Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 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, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,947.5	$ 1,608.0	$ 1,590.2	$ 2,377.4	$ 2,076.8	$ 1,643.7	$ 1,672.5	$ 2,286.5	$ 7,523.1	$ 7,679.5	$ 7,648.5
Operating income	363.1	310.9	314.6	542.8	289.8	302.7	330.8	545.1	1,531.4	1,468.4	1,776.1
Net income attributed to common shareholders	$ 243.9	$ 234.3	$ 235.7	$ 420.1	$ 205.0	$ 233.2	$ 231.0	$ 390.1	$ 1,134.0	$ 1,059.3	$ 1,203.7
Earnings Per Share, Basic
Earnings per common share (basic) (in dollars per share)	$ 0.77	$ 0.74	$ 0.75	$ 1.33	$ 0.65	$ 0.74	$ 0.73	$ 1.24	$ 3.60	$ 3.36	$ 3.81
Earnings Per Share (Diluted)
Earnings per common share (diluted) (in dollars per share)	$ 0.77	$ 0.74	$ 0.74	$ 1.33	$ 0.65	$ 0.74	$ 0.73	$ 1.23	$ 3.58	$ 3.34	$ 3.79