ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES AND OTHER LIABILITIES - Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES AND OTHER LIABILITIES
Patient prepayments, deposits and refunds payable	$ 24,187		$ 24,563
Insurance and self-insurance accruals	8,610		8,886
Accrued sales taxes and other taxes	7,283		6,810
Accrued professional fees	3,685		3,751
Education expenses	2,315
Derivative liability	1,205		724
Accrued interest payable	487		332
Other current liabilities	5,539		6,717
Total accrued expenses and other current liabilities	$ 53,311	$ 50,431	$ 51,783