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Revenue recognition - Contract Liabilities, Rollforward (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Movement in Deferred Revenue [Roll Forward]    
Beginning balance $ 12,938 $ 11,891
Additions 1,412 1,346
Deductions 0 0
ASC 606 Adjustment 0 (3,937)
Ending balance $ 14,350 $ 9,300