SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Patient Accounts Receivable Narrative (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Concentration Risk [Line Items]
Accounts receivable derived from Medicare	65.00%	64.00%
Net service revenue period of care payment rate (days)	30 days
Percentage of patient receivables outstanding	10.00%
Historical collection rate from Medicare	99.00%
Rate of request for anticipated payment submitted for the initial episode of care		20.00%