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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Stock Options Activity
The following table summarizes the Company’s stock option activity for 2018:
Shares
(in thousands)
Weighted
Average
Exercise
Price
Weighted
Average
Remaining
Contractual
Term (years)
Aggregate
Intrinsic
Value
(in thousands)
Outstanding at December 31, 20173,392 $14.41 
Granted889 $43.19 
Exercised(434)$16.84 
Canceled(103)$24.16 
Outstanding at December 31, 20183,744 $20.69 6.9$64,906 
Exercisable at December 31, 20182,415 $16.66 6.0$48,735 
Schedule of Restricted Stock Units Activity
The following table summarizes the Company’s restricted stock unit activity for 2018:
Shares
(in thousands)
Weighted Average Fair Value per Share at Grant Date
Nonvested at December 31, 2017259 $10.43 
Granted146 $42.76 
Vested(98)$10.74 
Canceled(18)$27.89 
Nonvested at December 31, 2018289 $25.56 
Assumptions used in the Black-Scholes Option-Pricing Model A summary of the assumptions used to estimate the fair values for the periods presented is as follows:
 Year Ended December 31,
 201820172016
Stock Options
Risk free interest rate2.3% to 3.0%1.8% to 2.3%1.1% to 2.1%
Expected term5.3 to 6.1 years5.1 to 6.1 years5.1 to 6.1 years
Expected volatility80.1% to 85.2%75.1% to 85.8%76.5% to 78.1%
Expected dividend yield—%  —%  —%  
Weighted-average fair value of option on grant date$30.87 $7.43 $6.69 
Employee Stock Purchase Plan
Risk free interest rate2.1% to 2.7%1.1% to 1.6%0.5% to 0.8%
Expected term0.5 to 1.0 years0.5 to 1.0 years0.5 to 1.0 years
Expected volatility44.7% to 113.1%54.8% to 152.8%59.5% to 71.3%
Expected dividend yield—%  —%  —%  
Schedule of Stock-Based Compensation, Activity The Company estimated the fair value of the modified stock options using the Black-Scholes model based on the following key assumptions:
Modification of Stock Options
Exercise price$13.32 
Common stock price on date of modification$49.60 
Expected term3.5 years
Expected volatility79.9%  
Expected dividend yield—%  
Stock-Based Compensation Expense
The following table summarizes the components of total stock-based compensation expense included in the consolidated statements of operations for the periods presented (in thousands):
 Year Ended December 31,
 201820172016
Cost of contract manufacturing$— $71 $386 
Research and development6,317 1,933 1,924 
Selling, general and administrative9,175 4,151 5,043 
Total$15,492 $6,155 $7,353