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Revenue recognition - Contract Liabilities, Rollforward (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Movement in Deferred Revenue [Roll Forward]    
Beginning balance $ 12,938 $ 11,891
Additions 2,864 2,586
Deductions 0 0
ASC 606 Adjustment 0 (3,937)
Ending balance $ 15,802 $ 10,540