Long-term debt - Summary of Current and Noncurrent Debt (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Debt Instrument [Line Items]    
Long-term Debt $ 11,897 $ 1,070
Less current portion (18) (336)
Total 11,879 734
Term Loan Credit Agreeement, Net of Debt Discount [Member]
   
Debt Instrument [Line Items]    
Long-term Debt 11,879 0
Wisconsin Economic Development Corporation - CDI [Member]
   
Debt Instrument [Line Items]    
Long-term Debt 0 377
Wisconsin Economic Development Corporation- SCP [Member]
   
Debt Instrument [Line Items]    
Long-term Debt 0 657
Licensing Agreements [Member]
   
Debt Instrument [Line Items]    
Long-term Debt 18 34
Other Long-term Debt Obligations [Member]
   
Debt Instrument [Line Items]    
Long-term Debt $ 0 $ 2