Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,933.4	$ 1,651.0	$ 1,548.7	$ 2,108.6	$ 1,947.5	$ 1,608.0	$ 1,590.2	$ 2,377.4	$ 7,241.7	$ 7,523.1	$ 7,679.5
Operating income	370.5	370.2	338.8	626.6	363.1	310.9	314.6	542.8	1,706.1	1,531.4	1,468.4
Net income attributed to common shareholders	$ 239.0	$ 266.8	$ 241.6	$ 452.5	$ 243.9	$ 234.3	$ 235.7	$ 420.1	$ 1,199.9	$ 1,134.0	$ 1,059.3
Earnings Per Share, Basic
Earnings per common share (basic) (in dollars per share)	$ 0.76	$ 0.85	$ 0.77	$ 1.43	$ 0.77	$ 0.74	$ 0.75	$ 1.33	$ 3.80	$ 3.60	$ 3.36
Earnings Per Share (Diluted)
Earnings per common share (diluted) (in dollars per share)	$ 0.76	$ 0.84	$ 0.76	$ 1.43	$ 0.77	$ 0.74	$ 0.74	$ 1.33	$ 3.79	$ 3.58	$ 3.34