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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Adoption of ASU 2016-09 - Adoption of New Revenue Standard (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, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 01, 2018
Impact of Recently Issued Accounting Standards                          
Net Revenues $ 56,056 $ 63,027 $ 58,229 $ 53,414 $ 55,596 $ 57,109 $ 54,578 $ 49,749   $ 230,726 $ 217,032 $ 202,245  
Retained earnings $ 257,516       $ 231,978         $ 257,516 $ 231,978    
Accounting Standards Update 2014-09 - Revenue from Contracts with Customers | Difference between Revenue Guidance in Effect before and after Topic 606                          
Impact of Recently Issued Accounting Standards                          
Retained earnings                         $ 4,900
Expected | Accounting Standards Update 2014-09 - Revenue from Contracts with Customers | Difference between Revenue Guidance in Effect before and after Topic 606                          
Impact of Recently Issued Accounting Standards                          
Net Revenues                 $ 0