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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-based compensation expense
Stock-based compensation expense is included in the consolidated statements of operations as follows (in thousands):
 
 
Year Ended December 31,
 
2018
 
2017
 
2016
Research and development
$
2,055

 
$
1,444

 
$
1,033

Selling, general and administrative
5,834

 
5,647

 
4,640

Total
$
7,889

 
$
7,091

 
$
5,673



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,
 
2018
 
2017
 
2016
Stock options
$
1,975

 
$
1,554

 
$
1,102

RSUs and RSAs
1,770

 
1,888

 
2,043

PSUs
1,511

 
1,792

 
2,528

PBOs
2,633

 
1,857

 
—

Total
$
7,889

 
$
7,091

 
$
5,673
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,
 
2018
 
2017
 
2016
Expected life (years)
5.6

 
5.4

 
5.3

Volatility
60.0
%
 
62.2
%
 
64.2
%
Risk-free interest rate
2.7
%
 
2.0
%
 
1.3
%
Expected dividend yield
0.0
%
 
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, 2015
3,918

 
$
4.49

Granted
971

 
$
4.16

Exercised
(398
)
 
$
2.60

Forfeited/Expired
(601
)
 
$
5.76

Outstanding at December 31, 2016
3,890

 
$
4.40

Granted
856

 
$
4.57

Exercised
(86
)
 
$
3.10

Forfeited/Expired
(81
)
 
$
7.72

Outstanding at December 31, 2017
4,579

 
$
4.40

Granted
645

 
$
9.56

Exercised
(772
)
 
$
5.56

Forfeited/Expired
(340
)
 
$
6.66

Outstanding at December 31, 2018
4,112

 
$
4.81

 
 
 
 
Exercisable at December 31, 2018
2,876

 
$
3.83

Vested and expected to vest at December 31, 2018
3,954

 
$
4.69

 
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, 2018
4,112

 
$
4.81

 
6.23
 
$
48,927

Exercisable at December 31, 2018
2,876

 
$
3.83

 
5.25
 
$
37,005

Vested and expected to vest at December 31, 2018
3,954

 
$
4.69

 
6.13
 
$
47,503
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, 2015
480

 
$
3.29

Granted
185

 
$
4.21

Vested
(435
)
 
$
3.40

Non-vested balance at December 31, 2016
230

 
$
3.82

Granted
143

 
$
4.75

Vested
(214
)
 
$
3.81

Non-vested balance at December 31, 2017
159

 
$
4.68

Granted
47

 
$
14.35

Vested
(151
)
 
$
4.71

Non-vested balance at December 31, 2018
55

 
$
12.83
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 January 1, 2016
545

 
$
3.15

Granted
330

 
$
4.10

Vested
(243
)
 
$
3.11

Forfeited/Expired
(15
)
 
$
2.74

Non-vested balance at December 31, 2016
617

 
$
3.69

Granted
275

 
$
4.22

Vested
(302
)
 
$
3.40

Forfeited/Expired
(30
)
 
$
4.12

Non-vested balance at December 31, 2017
560

 
$
4.08

Granted
86

 
$
10.56

Vested
(290
)
 
$
4.09

Forfeited/Expired
(8
)
 
$
4.73

Non-vested balance at December 31, 2018
348

 
$
5.66
Share-based compensation, performance shares sward outstanding activity
The following table summarizes PBO activities in 2018:
 
Number of Shares
 
Weighted Average Grant Date Fair Value Per Share
 
(in thousands)
 
 
Outstanding at December 31, 2016
—

 
$
—

Granted
1,720

 
$
2.54

Outstanding at December 31, 2017
1,720

 
$
2.54

Granted
1,200

 
$
5.02

Exercised
(84
)
 
$
2.54

Forfeited
(1,254
)
 
$
3.73

Outstanding at December 31, 2018
1,582

 
$
3.47


 
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, 2018
493

 
$
4.60

 
5.60
 
$
5,968

Vested and expected to vest at December 31, 2018
1,582

 
$
6.24

 
5.44
 
$
16,553

The following table summarizes PSU activities:
 
Number
of
Shares
 
Weighted Average
Grant Date
Fair Value
Per Share
 
(In Thousands)
 
 
Non-vested balance at January 1, 2016
989

 
$
2.94

Granted
629

 
$
4.10

Vested
(482
)
 
$
2.89

Forfeited/Expired
(305
)
 
$
2.82

Non-vested balance at January 1, 2017
831

 
$
3.88

Granted
276

 
$
4.25

Vested
(651
)
 
$
3.84

Forfeited/Expired
(27
)
 
$
3.65

Non-vested balance at January 1, 2018
429

 
$
4.20

Granted
306

 
$
6.71

Vested
(495
)
 
$
4.16

Non-vested balance at December 31, 2018
240

 
$
7.48
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,
 
2018
 
2017
 
2016
Expected life (years)
5.63

 
5.33

 
—

Volatility
60.3
%
 
62.3
%
 
—

Risk-free interest rate
5.6
%
 
5.3
%
 
—

Expected dividend yield
0.0
%
 
0.0
%
 
—