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Stock-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2019
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,
 201920182017
Research and development$1,562  $2,055  $1,444  
Selling, general and administrative5,381  5,834  5,647  
Total$6,943  $7,889  $7,091  
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,
 201920182017
Stock options$2,149  $1,975  $1,554  
RSUs and RSAs1,805  1,770  1,888  
PSUs1,087  1,511  1,792  
PBOs1,902  2,633  1,857  
Total$6,943  $7,889  $7,091  
Assumptions used to estimate the fair value of option grants The following summarize the ranges of weighted-average assumptions used to estimate the fair value of employee stock options granted:
 Year Ended December 31,
 201920182017
Expected life (years)5.65.65.4
Volatility55.3 %60.0 %62.2 %
Risk-free interest rate2.4 %2.7 %2.0 %
Expected dividend yield0.0 %0.0 %0.0 %
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, 20163,890  $4.40  
Granted
856  $4.57  
Exercised
(86) $3.10  
Forfeited/Expired
(81) $7.72  
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  

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, 20193,147  $6.31  5.6$31,961  
Exercisable at December 31, 20192,415  $4.23  4.7$28,406  
Vested and expected to vest at December 31, 20193,050  $6.05  5.5$31,600  
Schedule of share-based compensation, RSA activity
The following table summarizes RSA activities:
Number
of
Shares		Weighted Average
Grant Date
Fair Value
Per Share
(In Thousands) 
Non-vested balance at December 31, 2016230  $3.82  
Granted
143  $4.75  
Vested
(214) $3.81  
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  
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, 2016617  $3.69  
Granted
275  $4.22  
Vested
(302) $3.40  
Forfeited/Expired
(30) $4.12  
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  
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, 2016831  $3.88  
Granted
276  $4.25  
Vested
(651) $3.84  
Forfeited/Expired
(27) $3.65  
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  
The following table summarizes PBO activities:
Number of SharesWeighted Average Grant Date Fair Value Per Share
(in thousands) 
Outstanding at December 31, 2016—  $—  
Granted1,720  $2.54  
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  

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, 2019863  $5.57  7.3$8,993  
Vested and expected to vest at December 31, 20191,213  $8.47  7.7$10,181  

The total fair value of exercised PBOs were $1.3 million for 2019, $0.2 million for 2018 and zero for 2017. As of December 31, 2019, there was $0.9 million of unrecognized stock-based compensation cost related to non-vested PBOs, which we expect to recognize over a weighted average period of 0.8 years.
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,
 201920182017
Expected life (years)5.585.635.33
Volatility55.8 %60.3 %62.3 %
Risk-free interest rate2.5 %2.7 %2.0 %
Expected dividend yield0.0 %0.0 %0.0 %