| 
           Major customers (Details) 
         | 
        3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
| 
           Sep. 30, 2014
 
          entity 
         | 
        
           Sep. 30, 2013
 
          entity 
         | 
        
           Sep. 30, 2014
 
          entity 
         | 
        
           Sep. 30, 2013
 
          entity 
         | 
      |
| 
           Purchases | Vendor concentration risk | One major vendor 
         | 
        ||||
| Significant Customers | ||||
| Number of vendors | 1 | 1 | 1 | 1 | 
| Percentage of concentration risk | 17.30% | 10.40% | 13.60% | 10.00% | 
| 
           Net sales | Customer concentration risk 
         | 
        ||||
| Significant Customers | ||||
| Number of customers | 3 | 3 | 3 | 3 | 
| 
           Net sales | Customer one | Customer concentration risk 
         | 
        ||||
| Significant Customers | ||||
| Percentage of concentration risk | 16.80% | 15.00% | 16.70% | 15.00% | 
| 
           Net sales | Customer two | Customer concentration risk 
         | 
        ||||
| Significant Customers | ||||
| Percentage of concentration risk | 18.70% | 15.00% | 16.40% | 14.30% | 
| 
           Net sales | Customer three | Customer concentration risk 
         | 
        ||||
| Significant Customers | ||||
| Percentage of concentration risk | 10.60% | 11.80% | 10.90% | 11.90% | 
| 
           Net accounts receivable | Customer one | Customer concentration risk 
         | 
        ||||
| Significant Customers | ||||
| Percentage of concentration risk | 15.00% | |||
| 
           Net accounts receivable | Customer two | Customer concentration risk 
         | 
        ||||
| Significant Customers | ||||
| Percentage of concentration risk | 16.90% | |||
| 
           Net accounts receivable | Customer three | Customer concentration risk 
         | 
        ||||
| Significant Customers | ||||
| Percentage of concentration risk | 8.10% | |||