SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration of Credit Risk (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Concentration of Credit Risk
Cash, uninsured amount	$ 237.0
Revenue | Customer Concentration Risk | Centers for Medicare and Medicaid Services
Concentration of Credit Risk
Concentration risk (as a percent)	21.00%	29.00%	36.00%
Revenue | Customer Concentration Risk | UnitedHealthcare
Concentration of Credit Risk
Concentration risk (as a percent)	10.00%	13.00%	13.00%
Revenue | Customer Concentration Risk | State of Wisconsin
Concentration of Credit Risk
Concentration risk (as a percent)	12.00%	0.00%	0.00%
Accounts Receivable | Customer Concentration Risk | Centers for Medicare and Medicaid Services
Concentration of Credit Risk
Concentration risk (as a percent)	14.00%	19.00%	32.00%
Accounts Receivable | Customer Concentration Risk | UnitedHealthcare
Concentration of Credit Risk
Concentration risk (as a percent)	7.00%	7.00%	10.00%
Accounts Receivable | Customer Concentration Risk | State of Wisconsin
Concentration of Credit Risk
Concentration risk (as a percent)	22.00%	0.00%	0.00%