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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity
The following is a summary of stock option activity for the nine months ended September 30, 2018 (in thousands, except per share data):
Shares
(in thousands)
Weighted-
Average
Exercise
Price per Share
Outstanding at December 31, 20173,392 $14.41 
Granted
842 43.13 
Exercised
(290)16.45 
Canceled
(59)26.10 
Outstanding at September 30, 20183,885 $20.30 
Schedule of Restricted Stock Unit Activity
The following is a summary of restricted stock unit activity for the nine months ended September 30, 2018 (in thousands, except per share data):
Shares
(in thousands)
Weighted- Average Fair Value per Share at Grant Date 
Outstanding at December 31, 2017259 $10.43 
Granted
146 42.76 
Vested
(98)10.74 
Canceled
(8)29.74 
Outstanding at September 30, 2018299 $25.58 
Assumptions used in the Black-Scholes Option-Pricing Model
The fair value of the stock options granted or modified during the periods indicated was estimated using the Black-Scholes option pricing model, based on the following assumptions:

Three Months Ended September 30, Nine Months Ended September 30, 
2018201720182017
Risk free interest rate 2.8% to 2.9% 1.9% to 2.1% 2.3% to 2.9% 1.9% to 2.3% 
Expected term 3.5 to 6.1 years 6.1 years 3.5 to 6.1 years 5.1 to 6.1 years 
Expected volatility 79.9% to 83.2% 75.1% to 75.5% 79.9% to 85.2% 75.1% to 76.6% 
Expected dividend yield — %— %— %— %
Stock-Based Compensation Expense
The following table summarizes the components of total stock-based compensation expense included in the condensed consolidated statements of operations (in thousands):
 
Three Months Ended September 30, Nine Months Ended September 30, 
2018201720182017
Cost of contract manufacturing $— $— $— $71 
Research and development 3,275 294 5,018 1,313 
Selling, general and administrative 3,706 968 6,934 2,682 
Total $6,981 $1,262 $11,952 $4,066