NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounts Receivable and Cash and Cash Equivalents (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in the allowance for doubtful accounts
Beginning balance	$ 3.5	$ 1.2
Provision, charged to expense	1.2	3.7	$ 1.1
Write-off, less recoveries	(1.7)	(1.4)
Ending balance	3.0	3.5	$ 1.2
Money market account balance	$ 160.0	$ 72.9