SEGMENT AND GEOGRAPHICAL CONCENTRATION - Revenues by Product (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Revenue	$ 67,801	$ 62,653	$ 65,692	$ 61,885	$ 61,791	$ 59,310	$ 59,227	$ 56,306	$ 258,031	$ 236,634	$ 216,007
United States [Member]
Revenue from External Customer [Line Items]
Revenue									197,312	181,078	155,291
United States [Member] | Complex Spine [Member]
Revenue from External Customer [Line Items]
Revenue									77,529	71,915	63,398
United States [Member] | Minimally Invasive [Member]
Revenue from External Customer [Line Items]
Revenue									33,257	28,711	23,633
United States [Member] | Degenerative [Member]
Revenue from External Customer [Line Items]
Revenue									86,526	80,452	68,260
International [Member]
Revenue from External Customer [Line Items]
Revenue									$ 60,719	$ 55,556	$ 60,716