OPERATING REVENUES - OTHER OPERATING REVENUES (Details) - Other operating revenues - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Operating Revenues
Other operating revenues	$ 12.4	$ 20.4	$ 39.8	$ 47.6
Late payment charges
Disaggregation of Operating Revenues
Other operating revenues	6.0	9.7	18.1	34.9
Alternative revenues
Disaggregation of Operating Revenues
Other operating revenues	5.4	1.4	18.2	(17.2)
Other
Disaggregation of Operating Revenues
Other operating revenues	1.0	$ 9.3	3.5	$ 29.9
Late payment charges transferred to alternative revenues
Disaggregation of Operating Revenues
Other operating revenues	$ 1.7		$ 8.5