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Revenue - Capitalized Costs and Amortization (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Revenue    
Amortization period 7 years  
Revenue, Practical Expedient, Incremental Cost of Obtaining Contract [true/false] true  
Capitalized costs to obtain contracts, current $ 459 $ 413
Capitalized costs to obtain contracts, non-current 2,307 2,150
Amortization of capitalized costs to obtain contracts $ 109 $ 65