OPERATING REVENUES - OTHER OPERATING REVENUES (Details) - Other operating revenues - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Operating Revenues
Other operating revenues	$ 13.5	$ 12.4	$ 57.8	$ 39.8
Late payment charges
Disaggregation of Operating Revenues
Other operating revenues	11.9	6.0	44.2	18.1
Alternative revenues
Disaggregation of Operating Revenues
Other operating revenues	0.9	5.4	10.0	18.2
Other
Disaggregation of Operating Revenues
Other operating revenues	$ 0.7	$ 1.0	$ 3.6	$ 3.5