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Stock-based compensation and warrants - Summary of Stock-Based Compensation Expense by Award Type (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense $ 5,432us-gaap_AllocatedShareBasedCompensationExpense $ 2,493us-gaap_AllocatedShareBasedCompensationExpense
Stock Options [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense 4,755us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 2,477us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Restricted Stock Awards [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense   16us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Restricted Stock Units [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense 617us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		  
Employee Stock Purchase Plan [Member]    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Stock-based compensation expense $ 60us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember