Annual report pursuant to Section 13 and 15(d)

Industry, Segment and Geographic Information - Concentration (Details)

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Industry, Segment and Geographic Information - Concentration (Details) - customer
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Top five customers      
Significant Customers and Vendors      
Number of customers 5 5 5
Net sales | Customer concentration risk      
Significant Customers and Vendors      
Number of customers 2    
Net sales | SHI | Customer concentration risk      
Significant Customers and Vendors      
Percentage of concentration risk 23.00% 19.60% 19.00%
Net sales | CDW | Customer concentration risk      
Significant Customers and Vendors      
Percentage of concentration risk 19.40% 17.90% 17.90%
Net sales | Top five customers      
Significant Customers and Vendors      
Percentage of concentration risk 52.00% 48.00% 52.00%
Net accounts receivable | SHI | Customer concentration risk      
Significant Customers and Vendors      
Percentage of concentration risk 15.10%    
Net accounts receivable | CDW | Customer concentration risk      
Significant Customers and Vendors      
Percentage of concentration risk 28.60%    
Purchases | Vendor concentration risk | Solarwinds      
Significant Customers and Vendors      
Percentage of concentration risk 14.70% 10.80%  
Purchases | Vendor concentration risk | Sophos      
Significant Customers and Vendors      
Percentage of concentration risk 26.40% 23.10% 24.20%