GOODWILL AND OTHER INTANGIBLE ASSETS - Carrying Amount and Accumulated Amortization of Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	$ 483,059		$ 483,059
Total accumulated amortization	(140,680)		(132,007)
Total intangible assets, net	342,379		351,052
Amortization expense for intangible assets	8,700	$ 8,800
Referral sources
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	438,121		438,121
Total accumulated amortization	(117,048)		(110,498)
Trademarks/names
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	44,536		44,536
Total accumulated amortization	(23,247)		(21,146)
Other amortizable intangible assets
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	402		402
Total accumulated amortization	$ (385)		$ (363)