SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value of Financial Instruments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets and liabilities measured at fair value on a recurring basis
Transfers of financial assets between Level 1 and Level 2	$ 0
Transfers of financial assets between Level 2 and Level 1	0
Liabilities:
Long-term debt, gross	116,749,000	$ 125,300,000
Change in the contingent consideration liability
Beginning Balance	6,073,000	4,019,000
Payment of contingent consideration	(3,386,000)	(3,954,000)
Acquisitions		4,946,000
Change in fair value of contingent consideration	145,000	(210,000)
Accretion of contingent consideration	826,000	1,259,000
Unrealized gain related to currency translation	40,000	13,000
Ending Balance	$ 3,698,000	6,073,000
Minimum
Liabilities:
Discounted rate of cash outflow projections (as a percent)	14.50%
Incremental borrowing rate used to discount future cash flows from financial instruments (as a percent)	2.00%
Maximum
Liabilities:
Discounted rate of cash outflow projections (as a percent)	27.30%
Incremental borrowing rate used to discount future cash flows from financial instruments (as a percent)	5.20%
Level 3
Liabilities:
Fair value of outstanding borrowing	$ 116,500,000	124,900,000
Recurring
Assets:
Cash equivalents	303,000	22,000
Total	303,000	22,000
Liabilities:
Contingent consideration	3,698,000	6,073,000
Total	3,698,000	6,073,000
Recurring | Level 1
Assets:
Cash equivalents	303,000	22,000
Total	303,000	22,000
Recurring | Level 3
Liabilities:
Contingent consideration	3,698,000	6,073,000
Total	$ 3,698,000	$ 6,073,000