OPERATING REVENUES - OTHER OPERATING REVENUES (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Operating Revenues
Operating revenues	$ 2,377.4	$ 2,286.5
Other operating revenues
Disaggregation of Operating Revenues
Operating revenues	0.9	(3.5)
Other operating revenues | Alternative revenues
Disaggregation of Operating Revenues
Operating revenues	(19.7)	(16.1)
Other operating revenues | Late payment charges
Disaggregation of Operating Revenues
Operating revenues	13.2	11.4
Other operating revenues | Rental revenues
Disaggregation of Operating Revenues
Operating revenues	$ 7.4	$ 1.2