GOODWILL AND OTHER INTANGIBLE ASSETS - Carrying Amount and Accumulated Amortization of Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	$ 501,310		$ 501,310		$ 483,059
Total accumulated amortization	(149,635)		(149,635)		(132,007)
Total intangible assets, net	351,675		351,675		351,052
Amortization expense for intangible assets	9,000	$ 8,800	17,600	$ 17,600
Referral sources
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	456,372		456,372		438,121
Total accumulated amortization	(123,903)		(123,903)		(110,498)
Trademarks/names
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	44,536		44,536		44,536
Total accumulated amortization	(25,347)		(25,347)		(21,146)
Other amortizable intangible assets
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	402		402		402
Total accumulated amortization	$ (385)		$ (385)		$ (363)