SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Immaterial Error Correction (Details) - USD ($) $ in Thousands	3 Months Ended					6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Concentration Risk [Line Items]
Net revenue	$ 891,937				$ 781,609		$ 2,511,446	$ 2,227,897
Commercial:
Concentration Risk [Line Items]
Net revenue	769,524	$ 746,647	$ 648,507	$ 690,686	$ 678,706	$ 1,395,154	2,164,678	$ 1,927,426	$ 2,618,112
Commercial: | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
% of Revenue		86.80%	85.40%	85.90%	86.80%	86.10%		86.50%	86.40%
Government:
Concentration Risk [Line Items]
Net revenue	$ 106,645	$ 102,581	$ 97,841	$ 100,431	$ 93,114	$ 200,422	$ 307,067	$ 274,509	$ 374,940
Government: | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
% of Revenue		11.90%	12.90%	12.50%	11.90%	12.40%		12.40%	12.30%
As Previously Reported | Commercial:
Concentration Risk [Line Items]
Net revenue		$ 703,429	$ 611,434	$ 649,880	$ 644,385	$ 1,314,862		$ 1,893,105	$ 2,542,985
As Previously Reported | Commercial: | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
% of Revenue		81.80%	80.50%	80.80%	82.40%	81.20%		85.00%	83.90%
As Previously Reported | Government:
Concentration Risk [Line Items]
Net revenue		$ 145,799	$ 134,914	$ 141,237	$ 127,435	$ 280,714		$ 308,830	$ 450,067
As Previously Reported | Government: | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
% of Revenue		16.90%	17.80%	17.60%	16.30%	17.30%		13.90%	14.80%
Adjustment | Commercial:
Concentration Risk [Line Items]
Net revenue		$ 43,218	$ 37,073	$ 40,806	$ 34,321	$ 80,292		$ 34,321	$ 75,127
Adjustment | Commercial: | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
% of Revenue		5.00%	4.90%	5.10%	4.40%	4.90%		1.50%	2.50%
Adjustment | Government:
Concentration Risk [Line Items]
Net revenue		$ (43,218)	$ (37,073)	$ (40,806)	$ (34,321)	$ (80,292)		$ (34,321)	$ (75,127)
Adjustment | Government: | Revenue Benchmark | Company's Largest Payer
Concentration Risk [Line Items]
% of Revenue		(5.00%)	(4.90%)	(5.10%)	(4.40%)	(4.90%)		(1.50%)	(2.50%)