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Stock-based Compensation (Tables)
12 Months Ended
Dec. 31, 2020
Share-based Payment Arrangement [Abstract]  
Schedule of Stock-based Compensation Expense
Stock-based compensation expense is included in the consolidated statements of operations as follows (in thousands): 
 Year Ended December 31,
 202020192018
Research and development$1,620 $1,562 $2,055 
Selling, general and administrative6,108 5,381 5,834 
Total$7,728 $6,943 $7,889 

The following table presents total stock-based compensation expense by security type included in the consolidated statements of operations (in thousands):
 Year Ended December 31,
 202020192018
Stock options$2,381 $2,149 $1,975 
RSUs and RSAs2,231 1,805 1,770 
PSUs1,160 1,087 1,511 
PBOs1,956 1,902 2,633 
Total$7,728 $6,943 $7,889 
Assumptions Used to Estimate the Fair Value of Option Grants The following summarizes the weighted-average assumptions used to estimate the fair value of employee and non-employee stock options granted:
 Year Ended December 31,
 202020192018
Expected life (years)5.35.65.6
Volatility50.4 %55.3 %60.0 %
Risk-free interest rate1.0 %2.4 %2.7 %
Expected dividend yield0.0 %0.0 %0.0 %
The following summarizes the weighted-average assumptions used to estimate the fair value of 76,000 shares of stock options granted to non-employees during the year ended December 31, 2020 for services valued at $0.4 million:
Expected life (years)5.4
Volatility51.6 %
Risk-free interest rate0.4 %
Expected dividend yield0.0 %
Schedule of Share-based Compensation, Stock Options, Activity
The following tables summarizes stock option activities:
Number
of
Shares
Weighted
Average
Exercise Price Per Share
(In Thousands)
Outstanding at December 31, 20174,579$4.40 
Granted
645$9.56 
Exercised
(772)$5.56 
Forfeited/Expired
(340)$6.66 
Outstanding at December 31, 20184,112$4.81 
Granted406$20.68 
Exercised(1,045)$4.50 
Forfeited/Expired(326)$11.01 
Outstanding at December 31, 20193,147$6.31 
Granted496$13.30 
Exercised(210)$6.30 
Forfeited/Expired(48)$16.71 
Outstanding at December 31, 20203,385$7.19 

Number
of
Shares
Weighted
Average
Exercise Price Per Share
Weighted
Average
Remaining
Contractual Term
Aggregate Intrinsic
Value
(In Thousands)(In Years)(In Thousands)
Outstanding at December 31, 20203,385 $7.19 5.4$49,542 
Exercisable at December 31, 20202,569 $5.09 4.3$42,998 
Vested and expected to vest at December 31, 20203,279 $6.96 5.3$48,786 
Schedule of Share-based Compensation, RSA Activity
The following table summarizes RSA activities:
Number
of
Shares
Weighted Average
Grant Date
Fair Value
Per Share
Non-vested balance at December 31, 2017159$4.68 
Granted
47$14.35 
Vested
(151)$4.71 
Non-vested balance at December 31, 201855$12.83 
Granted40$17.18 
Vested(56)$12.83 
Forfeited/Expired(4)$17.18 
Non-vested balance at December 31, 201935$17.18 
Granted96$11.44 
Vested(35)$17.18 
Non-vested balance at December 31, 202096$11.44 
Schedule of Share-based Compensation, RSU Activity
The following table summarizes RSU activities:
Number
of
Shares
Weighted Average
Grant Date
Fair Value
Per Share
(In Thousands)
Non-vested balance at December 31, 2017560$4.08 
Granted
86$10.56 
Vested
(290)$4.09 
Forfeited/Expired
(8)$4.73 
Non-vested balance at December 31, 2018348$5.66 
Granted72$19.19 
Vested(210)$5.03 
Forfeited/Expired(9)$13.60 
Non-vested balance at December 31, 2019201$10.76 
Granted156$14.22 
Vested(168)$10.05 
Forfeited/Expired(13)$15.16 
Non-vested balance at December 31, 2020176$14.17 
Share-based Compensation, Performance Shares Award Outstanding Activity
The following table summarizes PSU activities:
Number
of
Shares
Weighted Average
Grant Date
Fair Value
Per Share
(In Thousands)
Non-vested balance at December 31, 2017429$4.20 
Granted
306$6.71 
Vested
(495)$7.16 
Non-vested balance at December 31, 2018240$7.48 
Granted95$14.98 
Vested(200)$6.58 
Forfeited/Expired
(15)$15.58 
Non-vested balance at December 31, 2019120$13.88 
Granted124$13.59 
Vested(107)$11.28 
Forfeited/Expired
(6)$21.80 
Non-vested balance at December 31, 2020131$15.34 
The following tables summarizes PBO activities:
Number of SharesWeighted Average Grant Date Fair Value Per Share
(in thousands)
Outstanding at December 31, 20171,720$2.54 
Granted
1,200$5.02 
Exercised(84)$2.54 
Forfeited(1,254)$3.73 
Outstanding at December 31, 20181,582$3.47 
Granted
718$11.44 
Exercised
(422)$3.17 
Forfeited
(618)$10.34 
Outstanding at December 31, 20191,260$4.75 
Granted
689$6.37 
Forfeited
(389)$6.42 
Outstanding at December 31, 20201,560$5.05 

Number
of
Shares
Weighted
Average
Exercise Price Per Share
Weighted
Average
Remaining
Contractual Term
Aggregate Intrinsic
Value
(In Thousands)(In Years)(In Thousands)
Exercisable at December 31, 20201,156 $7.55 6.6$16,504 
Vested and expected to vest at December 31, 20201,510 $9.54 7.2$18,567 
Schedule of Assumptions Used The following summarize the ranges of weighted-average assumptions used to estimate the fair value of employee stock options granted:
 Year Ended December 31,
 202020192018
Expected life (years)5.35.65.6
Volatility49.9 %55.8 %60.3 %
Risk-free interest rate1.3 %2.5 %2.7 %
Expected dividend yield0.0 %0.0 %0.0 %