OPERATING REVENUES - OTHER OPERATING REVENUES (Details) - Other operating revenues - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Operating Revenues
Other operating revenues	$ 21.9	$ 5.7	$ 44.3	$ 27.4
Late payment charges
Disaggregation of Operating Revenues
Other operating revenues	17.3	0.0	32.3	12.1
Alternative revenues
Disaggregation of Operating Revenues
Other operating revenues	2.9	4.3	9.1	12.8
Other
Disaggregation of Operating Revenues
Other operating revenues	$ 1.7	$ 1.4	$ 2.9	$ 2.5