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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	$ 483,526		$ 483,526		$ 293,943
Total accumulated amortization	(88,448)		(88,448)		(74,230)
Total intangible assets, net	395,078		395,078		219,713
Amortization expense for intangible assets	7,500	$ 4,900	$ 17,300	$ 14,700
Weighted average amortization period of intangibles			15 years 11 months
Referral sources
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	438,445		$ 438,445		257,792
Total accumulated amortization	(77,749)		$ (77,749)		(63,353)
Weighted average amortization period of intangibles			17 years 1 month
Trademarks/names
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	44,702		$ 44,702		32,000
Total accumulated amortization	(10,657)		$ (10,657)		(8,000)
Weighted average amortization period of intangibles			4 years 1 month
Other amortizable intangible assets
Finite-Lived Intangible Assets [Line Items]
Total gross intangible assets	379		$ 379		4,151
Total accumulated amortization	$ (42)		$ (42)		$ (2,877)
Weighted average amortization period of intangibles			1 year 6 months