REVENUE RECOGNITION - Narrative (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment product_category	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings (accumulated deficit)	$ (2,072)				$ (2,072)				$ (2,072)	$ (2,072)
Sales	97,374	$ 88,828	$ 91,266	$ 87,342	88,286	$ 89,301	$ 81,344	$ 83,098	364,810	342,029	$ 341,159
Increase in operating income	$ (1,229)	$ 3,513	$ 2,186	$ 881	$ (2,841)	$ 2,154	$ 23	$ 2,051	$ 5,351	1,387	10,039
Refund period									90 days
Sales returns									$ 1,700	1,600	1,400
Number of principal categories of products | product_category									6
Number of business segments | segment									4
NSP Americas; NSP Russia, Central and Eastern Europe; and NSP China
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Number of operating segments | segment									3
Synergy WorldWide
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Sales									$ 141,028	123,833	124,793
Number of operating segments | segment									1
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retained earnings (accumulated deficit)												$ 900
Sales									$ 200
Increase in operating income									47
Shipping and Handling
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Sales									$ 7,100	$ 8,200	$ 9,200