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Summary of Significant Accounting Policies - Trade Accounts Receivable (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Jan. 02, 2018
Dec. 31, 2017
Dec. 31, 2016
Trade Accounts Receivable        
Trade accounts receivable $ 36,428   $ 32,579  
Unbilled accounts receivable 7,577      
Trade and unbilled accounts receivable 44,005   32,579  
Less allowance for doubtful accounts (211)   (48) $ (124)
Accounts receivable, net $ 43,794 $ 38,522 $ 32,531