SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Immaterial Error Correction (Details) - USD ($) $ in Thousands	3 Months Ended					6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2020
Concentration Risk [Line Items]
Net revenue	$ 860,272				$ 740,848	$ 1,619,509	$ 1,446,288
Commercial payers
Concentration Risk [Line Items]
Net revenue	$ 746,647	$ 648,507	$ 690,686	$ 678,706	642,726	$ 1,395,154	1,248,720	$ 1,927,426	$ 2,618,112
Commercial payers | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
Concentration risk	86.80%	85.40%	85.90%	86.80%		86.10%		86.50%	86.40%
Government payers
Concentration Risk [Line Items]
Net revenue	$ 102,581	$ 97,841	$ 100,431	$ 93,114	$ 95,124	$ 200,422	$ 181,395	$ 274,509	$ 374,940
Government payers | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
Concentration risk	11.90%	12.90%	12.50%	11.90%		12.40%		12.40%	12.30%
Previously Reported | Commercial payers
Concentration Risk [Line Items]
Net revenue	$ 703,429	$ 611,434	$ 649,880	$ 644,385		$ 1,314,862		$ 1,893,105	$ 2,542,985
Previously Reported | Commercial payers | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
Concentration risk	81.80%	80.50%	80.80%	82.40%		81.20%		85.00%	83.90%
Previously Reported | Government payers
Concentration Risk [Line Items]
Net revenue	$ 145,799	$ 134,914	$ 141,237	$ 127,435		$ 280,714		$ 308,830	$ 450,067
Previously Reported | Government payers | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
Concentration risk	16.90%	17.80%	17.60%	16.30%		17.30%		13.90%	14.80%
Revision of Prior Period, Adjustment | Commercial payers
Concentration Risk [Line Items]
Net revenue	$ 43,218	$ 37,073	$ 40,806	$ 34,321		$ 80,292		$ 34,321	$ 75,127
Revision of Prior Period, Adjustment | Commercial payers | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
Concentration risk	5.00%	4.90%	5.10%	4.40%		4.90%		1.50%	2.50%
Revision of Prior Period, Adjustment | Government payers
Concentration Risk [Line Items]
Net revenue	$ (43,218)	$ (37,073)	$ (40,806)	$ (34,321)		$ (80,292)		$ (34,321)	$ (75,127)
Revision of Prior Period, Adjustment | Government payers | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
Concentration risk	(5.00%)	(4.90%)	(5.10%)	(4.40%)		(4.90%)		(1.50%)	(2.50%)