| 
           Major customers (Details) 
         | 
        3 Months Ended | |
|---|---|---|
| 
           Mar. 31, 2014
 
          item 
         | 
        
           Mar. 31, 2013
 
          item 
         | 
      |
| 
           Purchases | Vendor concentration risk | One major vendor 
         | 
        ||
| Significant Customers | ||
| Number of vendors | 1 | 1 | 
| Percentage of concentration risk | 10.43% | 10.40% | 
| 
           Net sales | Customer concentration risk 
         | 
        ||
| Significant Customers | ||
| Number of customers | 3 | 3 | 
| 
           Net sales | Customer one | Customer concentration risk 
         | 
        ||
| Significant Customers | ||
| Percentage of concentration risk | 15.70% | 14.50% | 
| 
           Net sales | Customer two | Customer concentration risk 
         | 
        ||
| Significant Customers | ||
| Percentage of concentration risk | 15.60% | 14.40% | 
| 
           Net sales | Customer three | Customer concentration risk 
         | 
        ||
| Significant Customers | ||
| Percentage of concentration risk | 10.50% | 11.30% | 
| 
           Net accounts receivable | Customer one | Customer concentration risk 
         | 
        ||
| Significant Customers | ||
| Percentage of concentration risk | 14.20% | |
| 
           Net accounts receivable | Customer two | Customer concentration risk 
         | 
        ||
| Significant Customers | ||
| Percentage of concentration risk | 14.10% | |
| 
           Net accounts receivable | Customer three | Customer concentration risk 
         | 
        ||
| Significant Customers | ||
| Percentage of concentration risk | 9.30% | |