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Business and Summary of Significant Accounting Policies - Concentrations (Details)
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
One distributor | Revenue | Customer concentration risk          
Concentration Risk [Line Items]          
Concentration risk, percentage 43.00% 46.00% 43.00% 46.00%  
One distributor | Accounts receivable | Customer concentration risk          
Concentration Risk [Line Items]          
Concentration risk, percentage     42.00%   46.00%
Channel network | Revenue | Sales method risk          
Concentration Risk [Line Items]          
Concentration risk, percentage 90.00% 87.00% 90.00% 87.00%