GOODWILL AND DEFINITE-LIVED INTANGIBLE ASSETS - Definite-lived intangible assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
DEFINITE-LIVED INTANGIBLE ASSETS
Gross Carrying Amount	$ 360,549		$ 360,549		$ 384,039
Accumulated Amortization	(165,276)		(165,276)		(143,229)
Net Carrying Amount	195,273		195,273		$ 240,810
Amortization expense	13,753	$ 17,160	27,558	$ 34,170
Impairment	17,979		$ 17,979
Customer relationships
DEFINITE-LIVED INTANGIBLE ASSETS
Weighted Average Amortization Period (in years)			9 years 6 months		9 years 9 months 18 days
Gross Carrying Amount	88,000		$ 88,000		$ 100,200
Accumulated Amortization					(1,238)
Net Carrying Amount	88,000		$ 88,000		$ 98,962
Patient relationships
DEFINITE-LIVED INTANGIBLE ASSETS
Weighted Average Amortization Period (in years)			5 years 4 months 24 days		5 years 10 months 24 days
Gross Carrying Amount	170,100		$ 170,100		$ 170,100
Accumulated Amortization	(77,025)		(77,025)		(67,964)
Net Carrying Amount	93,075		$ 93,075		$ 102,136
Trade names and trademarks
DEFINITE-LIVED INTANGIBLE ASSETS
Weighted Average Amortization Period (in years)			1 year 6 months		1 year 9 months 18 days
Gross Carrying Amount	29,350		$ 29,350		$ 30,650
Accumulated Amortization	(23,593)		(23,593)		(20,270)
Net Carrying Amount	5,757		$ 5,757		$ 10,380
Non-compete employment agreements
DEFINITE-LIVED INTANGIBLE ASSETS
Weighted Average Amortization Period (in years)			1 year 3 months 18 days		1 year 7 months 6 days
Gross Carrying Amount	51,399		$ 51,399		$ 61,389
Accumulated Amortization	(42,958)		(42,958)		(44,100)
Net Carrying Amount	8,441		8,441		$ 17,289
Physician relationships
DEFINITE-LIVED INTANGIBLE ASSETS
Weighted Average Amortization Period (in years)					4 years 9 months 18 days
Gross Carrying Amount	21,700		21,700		$ 21,700
Accumulated Amortization	$ (21,700)		$ (21,700)		(9,657)
Net Carrying Amount					$ 12,043