ORGANIZATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Effect of Topic 606 on Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Condensed Consolidated Statement of Operations
Net revenues	$ 266,966	$ 263,386	$ 500,961	$ 497,067
Other operating costs	30,536	31,861	61,632	64,550
Income from operations	20,329	16,464	20,952	6,923
Income (loss) before income taxes	12,836	2,189	(15,978)	(21,545)
Net income (loss)	12,928	1,637	(9,690)	(16,097)
Comprehensive income (loss)	15,268	1,620	(9,932)	(16,131)
Before adoption of ASC 606 | ASU 2014-09
Condensed Consolidated Statement of Operations
Net revenues	268,288		503,181
Other operating costs	31,868		63,832
Income from operations	20,319		20,972
Income (loss) before income taxes	12,826		(15,958)
Net income (loss)	12,918		(9,670)
Comprehensive income (loss)	15,258		(9,912)
Adjustments | ASU 2014-09
Condensed Consolidated Statement of Operations
Net revenues	1,322		2,220
Other operating costs	1,332		2,200
Income from operations	(10)		20
Income (loss) before income taxes	(10)		20
Net income (loss)	(10)		20
Comprehensive income (loss)	(10)		20
Cranial revenue from satisfying performance obligations coming from previous period					$ 1,000
Estimated uncollectible amounts due from self-pay patients	1,300		2,200
Patient Care
Condensed Consolidated Statement of Operations
Net revenues	218,158	216,221	406,665	403,858
Income from operations	35,020	35,154	52,113	49,698
Income (loss) before income taxes	28,564	27,030	37,668	33,412
Net income (loss)	28,564	27,030	37,668	33,412
Patient Care | Before adoption of ASC 606
Condensed Consolidated Statement of Operations
Net revenues		$ 216,221		403,858
Patient Care | Adjustments | ASU 2014-09
Condensed Consolidated Statement of Operations
Net revenues	$ (1,300)			$ (2,200)
Cranial revenue deferred			$ 1,000