REVENUE - Net Revenue Earned by Category of Payer (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Net revenue	$ 720,779	$ 615,880	$ 497,266	$ 476,492	$ 505,730	$ 493,928	$ 479,490	$ 460,643	$ 2,310,417	$ 1,939,791	$ 1,828,046
Commercial payers
Disaggregation of Revenue [Line Items]
Net revenue									2,001,105	1,699,450	1,598,703
Government payers
Disaggregation of Revenue [Line Items]
Net revenue									285,128	217,876	203,651
Patients
Disaggregation of Revenue [Line Items]
Net revenue									$ 24,184	$ 22,465	$ 25,692