REVENUE - Net Revenue Earned by Category of Payer (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
Disaggregation of Revenue [Line Items]
Net revenue	$ 891,937				$ 781,609		$ 2,511,446	$ 2,227,897
Commercial payers
Disaggregation of Revenue [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
Government payers
Disaggregation of Revenue [Line Items]
Net revenue	106,645	$ 102,581	$ 97,841	$ 100,431	93,114	$ 200,422	307,067	274,509	$ 374,940
Patients
Disaggregation of Revenue [Line Items]
Net revenue	$ 15,768				$ 9,789		$ 39,701	$ 25,962