FAIR VALUE MEASUREMENTS - (Narrative) (Details) $ in Millions	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Discount rate | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate applied	0.100
Discount rate | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate applied	0.150
Discount rate | Weighted average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate applied	0.107
Money market funds | Restricted cash and cash equivalents
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Settlement funds	$ 17.8	$ 26.5