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Goodwill and Intangible Assets - NYCDA Acquisition (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Goodwill [Line Items]        
Goodwill $ 20,744 $ 17,919 $ 20,744 $ 20,744
New York Code And Design Academy, Inc.        
Goodwill [Line Items]        
Cash paid in acquisition 13,500      
Contingent consideration 11,500      
Goodwill $ 13,900