GOODWILL AND OTHER INTANGIBLE ASSETS - Carrying Amount and Accumulated Amortization of Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	$ 501,310		$ 501,310		$ 483,059
Total accumulated amortization	(157,546)		(157,546)		(132,007)
Total intangible assets, net	343,764		343,764		351,052
Amortization expense for intangible assets	7,900	$ 8,800	25,500	$ 26,300
Referral sources
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	456,372		456,372		438,121
Total accumulated amortization	(130,758)		(130,758)		(110,498)
Trademarks/names
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	44,536		44,536		44,536
Total accumulated amortization	(26,403)		(26,403)		(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)