QUARTERLY FINANCIAL INFORMATION (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,963.0	$ 1,712.5	$ 1,602.0	$ 2,194.8	$ 1,848.3	$ 1,698.7	$ 991.2	$ 1,387.9	$ 7,472.3	$ 5,926.1	$ 4,997.1
Operating Income (Loss)	361.7	399.0	332.1	589.3	380.2	345.7	165.8	358.8	1,682.1	1,250.5	1,112.1
Net income attributed to common shareholders	$ 194.4	$ 217.0	$ 181.4	$ 346.2	$ 179.3	$ 182.5	$ 80.9	$ 195.8	$ 939.0	$ 638.5	$ 588.3
Earnings Per Share, Basic
Earnings per common share (basic) (in dollars per share)	$ 0.62	$ 0.69	$ 0.57	$ 1.10	$ 0.57	$ 0.58	$ 0.36	$ 0.87	$ 2.98	$ 2.36	$ 2.61
Earnings Per Share (Diluted)
Earnings per common share (diluted) (in dollars per share)	$ 0.61	$ 0.68	$ 0.57	$ 1.09	$ 0.57	$ 0.58	$ 0.35	$ 0.86	$ 2.96	$ 2.34	$ 2.59