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Quarterly Financial Information (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 $ 2,055.0 $ 1,657.5 $ 1,631.5 $ 2,304.5 $ 1,963.0 $ 1,712.5 $ 1,602.0 $ 2,194.8 $ 7,648.5 $ 7,472.3 $ 5,926.1
Operating income 412.1 393.6 362.2 617.3 361.7 399.0 332.1 589.3 1,785.2 1,682.1 1,250.5
Net income attributed to common shareholders $ 432.6 $ 215.4 $ 199.1 $ 356.6 $ 194.4 $ 217.0 $ 181.4 $ 346.2 $ 1,203.7 $ 939.0 $ 638.5
Earnings Per Share, Basic                      
Earnings per common share (basic) (in dollars per share) $ 1.37 $ 0.68 $ 0.63 $ 1.13 $ 0.62 $ 0.69 $ 0.57 $ 1.10 $ 3.81 $ 2.98 $ 2.36
Earnings Per Share (Diluted)                      
Earnings per common share (diluted) (in dollars per share) $ 1.36 $ 0.68 $ 0.63 $ 1.12 $ 0.61 $ 0.68 $ 0.57 $ 1.09 $ 3.79 $ 2.96 $ 2.34