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Summary of Significant Accounting Policies - Revenue Recognition Narrative (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Sep. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Revenue from External Customer [Line Items]        
Period to accept returned unused product after product expiration     1 year  
Increase in product returns estimate $ 2,408zgnx_IncreaseDecreaseAccruedProductReturnsEstimate $ 1,226zgnx_IncreaseDecreaseAccruedProductReturnsEstimate    
Change in estimate, effect on earnings per share       $ 0.03zgnx_ChangeinEstimateEffectofChangeonBasicEarningsPerShare
Sales revenue, discount rate     2.00%zgnx_SalesRevenueDiscountRatePercent  
Zohydro ER and Sumavel DosePro [Member]        
Revenue from External Customer [Line Items]        
Period to accept returned unused product before expiration     6 months  
Period to accept returned unused product after product expiration     12 months  
Sumavel DosePro [Member]        
Revenue from External Customer [Line Items]        
Product shelf life     30 months  
Period to accept returned unused product before expiration     6 months  
Period to accept returned unused product after product expiration     1 year