Summary of significant accounting policies (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) item issuer instrument	Dec. 31, 2015 USD ($)
Concentrations of Credit Risk
Number of issuer to which company limits the amount of credit exposure other than US Government guaranteed securities | issuer	1
Number of financial investment to which company limits the amount of credit exposure other than US Government guaranteed securities | instrument	1
Accounts Receivable
Allowance for doubtful accounts | $	$ 0	$ 0
Inventory
Shelf life for finished goods inventory, maximum	36 months
Variable Interest Entities
Number of entities in which variable interest held | item	0
Minimum
Property and Equipment
Estimated useful life	3 years
Maximum
Property and Equipment
Estimated useful life	5 years