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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [Abstract]  
Summary of Stock-Based Compensation Expense Stock-based compensation expense included within “total operating costs and expenses” for the years ended December 31, 2019, 2018, and 2017, was as follows:
 
Year Ended December 31,
 
2019
 
2018
 
2017
Selling, general and administrative
$
14,118

 
$
9,268

 
$
9,178

Research and development
4,316

 
2,566

 
1,885

Total stock-based compensation
$
18,434

 
$
11,834

 
$
11,063
Fair Value of Stock Options Granted Using Black-Scholes Option Pricing Model
The fair value of stock options granted was estimated at the date of grant using the Black-Scholes option-pricing model. The following assumptions were used to determine fair value for the stock awards granted in the applicable year:
 
Year Ended December 31,
 
2019
 
2018
 
2017
Expected option life (in years) (a)
5.34
 
4.73
 
4.84
Risk-free interest rate (b)
1.47% - 2.49%
 
1.81% - 2.75%
 
0.82% - 1.90%
Volatility (c)
61.6% - 76.1%
 
50.0% - 56.2%
 
49.3% - 61.4%
Dividend yield (d)
—%
 
—%
 
—%
Weighted-average grant-date fair value per stock option
$5.85
 
$8.64
 
$2.89
(a)
Determined by the historical stock option exercise behavior of our employees (maximum term is 10 years).
(b)
Based upon the U.S. Treasury yields in effect during the period which the options were granted (for a period equaling the stock options’ expected term).
(c)
Measured using our historical stock price for a period equal to stock options’ expected term.
(d)
We do not expect to declare any cash dividends in the foreseeable future.
Summary of Stock Option Activity
Stock option activity during the years ended December 31, 2019, 2018, and 2017 was as follows:

Number of
Shares
 
Weighted-
Average
Exercise
Price/Share
 
Weighted-
Average
Remaining
Contractual
Term (Years)
 
Aggregate
Intrinsic
Value
 

Outstanding — December 31, 2016
14,340,582

 
$
6.86

 

 

 

Granted
1,223,483

 
6.51

 

 

 

Exercised
(937,482
)
 
6.40

 

 
$
6,813

 
(1
)
Forfeited
(244,793
)
 
6.26

 

 

 

Expired
(524,577
)
 
6.27

 

 

 

Outstanding — December 31, 2017
13,857,213

 
6.89

 

 

 

Granted
1,094,358

 
18.87

 

 

 

Exercised
(7,858,141
)
 
6.74

 

 
$
84,758

 
(1
)
Forfeited
(214,464
)
 
6.54

 

 

 

Expired
(35,381
)
 
6.94

 

 

 

Outstanding — December 31, 2018
6,843,585

 
8.98

 

 


 

Granted
1,113,081

 
10.54

 

 

 

Exercised
(1,121,403
)
 
6.38

 

 
$
2,919

 
(1
)
Forfeited
(172,074
)
 
9.84

 

 

 

Expired
(223,253
)
 
11.09

 

 

 

Outstanding — December 31, 2019
6,439,936

 
$
9.61

 
6.16
 
$
1

 
(2
)
Vested (exercisable) — December 31, 2019
4,682,766

 
$
8.66

 
5.29
 
$
1

 
(2
)
Unvested (unexercisable) — December 31, 2019
1,757,170

 
$
12.14

 
8.47
 
$
—

 
(2
)

(1)
Represents the total difference between our closing stock price at the time of exercise and the stock option exercise price, multiplied by the number of options exercised.
(2)
Represents the total difference between our closing stock price on the last trading day of 2019 and the stock option exercise price, multiplied by the number of in-the-money options as of December 31, 2019. The amount of intrinsic value will change based on the fair market value of our stock.
Summary of Stock Option Grants
The following table summarizes information with respect to stock option grants as of December 31, 2019:
 
Outstanding
 
Exercisable
Exercise Price
Granted Stock
Options
Outstanding
 
Weighted-
Average
Remaining
Contractual
Life (Years)
 
Weighted-
Average
Exercise
Price
 
Granted
Stock
Options
Exercisable
 
Weighted-
Average
Exercise
Price
$1.47 - 4.96
91,599

 
2.80
 
$
4.16

 
82,428

 
$
4.15

$4.97 - 6.91
2,210,050

 
6.18
 
6.00

 
1,866,277

 
5.97

$6.92 - 9.00
1,801,136

 
4.91
 
7.84

 
1,565,804

 
7.76

$9.01 - 12.00
1,135,793

 
7.23
 
11.03

 
509,250

 
10.70

$12.01 - 22.64
1,201,358

 
7.24
 
17.97

 
659,007

 
17.39


6,439,936

 
6.16
 
$
9.61

 
4,682,766

 
$
8.66
Summary of Restricted Stock Award and Restricted Stock Units Activity
A summary of restricted stock award activity is as follows:
 
Number of
Restricted Stock
Awards
 
Weighted Average
Fair Value per
Share at Grant
Date
Unvested — December 31, 2016
2,152,157

 
$
6.29

Granted
927,306

 
6.22

Vested
(1,137,555
)
 
6.38

Forfeited
(378,990
)
 
5.95

Unvested — December 31, 2017
1,562,918

 
6.27

Granted
1,092,534

 
17.35

Vested
(635,320
)
 
6.51

Forfeited
(218,002
)
 
7.56

Unvested — December 31, 2018
1,802,130

 
12.75

Granted
1,091,353

 
10.50

Vested
(972,404
)
 
11.70

Forfeited
(261,320
)
 
11.48

Unvested — December 31, 2019
1,659,759

 
$
11.67


A summary of restricted stock unit activity is as follows:

Number of
Restricted Stock
Units

Weighted Average
Fair Value per
Share at Grant
Date
Outstanding — December 31, 2017
217,206


$
6.49

Granted
138,334


28.31

Market-based achievement adjustment at vesting
100,326


6.49

Share issuance upon approved achievement date
(200,652
)

6.49

Outstanding — December 31, 2018
255,214


17.91

Granted
257,585


12.87

Market-based achievement adjustment at vesting
116,880


6.49

Share issuance
(243,760
)

7.04

Outstanding — December 31, 2019
385,919


$
18.00
Fair Value of Restricted Stock and Restricted Stock Units Awards
 
Year Ended December 31,

2019
 
2018
 
2017
Restricted stock award expense
$
9,170

 
$
5,180

 
$
4,985



2019

2018

2017
Restricted stock unit expense
$
3,019


$
2,301


$
1,030
Issued Shares of Common Stock
We issued shares of common stock to our employees in connection with our 401(k) program, partially matching our employees’ annual 401(k) contributions, as summarized below:
 
Year Ended December 31,

2019
 
2018
 
2017
Shares of common stock issued
225,780

 
70,379

 
102,874

Value of employer match in shares
$
1,289

 
$
762

 
$
639