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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Advertising Costs and Operating Leases (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Advertising costs $ 0.1 $ 0.1 $ 0.1
Maximum      
Initial operating lease terms 5 years