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Stock-Based Compensation
 12 Months Ended
Dec. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-Based Compensation Stock-Based Compensation
We recognize total stock-based compensation expense in our Condensed Consolidated Statements of Operations as follows (in thousands):
 
Year Ended December 31,
 
2019
 
2018
 
2017
Cost of goods sold
$
4,294

 
$
4,629

 
$
2,333

Research and development
63,224

 
56,193

 
21,252

General and administrative
32,277

 
27,279

 
13,030

Total stock-based compensation
$
99,795

 
$
88,101

 
$
36,615


As of December 31, 2019, total unrecognized compensation costs of $213.7 million related to unvested stock-based compensation arrangements are expected to be recognized as expense over a weighted-average period of 1.75 years.
Stock-based compensation expense resulting from our ESPP was not significant in the years ended December 31, 2019, 2018, and 2017.
Black-Scholes Assumptions
The following table lists the Black-Scholes option-pricing model assumptions used to calculate the fair value of employee and director stock options, as well as the resulting grant-date fair value:
 
Year Ended December 31,
 
2019
 
2018
 
2017
Average risk-free interest rate
1.8
%
 
2.8
%
 
2.0
%
Dividend yield
0.0
%
 
0.0
%
 
0.0
%
Average volatility factor
62.2
%
 
61.0
%
 
54.2
%
Weighted-average expected life
5.6 years

 
5.1 years

 
5.3 years

Weighted-average grant-date fair value of options granted
$
12.25

 
$
29.86

 
$
20.08


The average risk-free interest rate is based on the U.S. treasury yield curve in effect at the time of grant for periods commensurate with the expected life of the stock-based award. We have never paid dividends, nor do we expect to pay dividends in the foreseeable future; therefore, we used a dividend yield of zero. Our estimate of expected volatility is based on the daily historical trading data of our common stock at the time of grant over a historical period commensurate with the expected life of the stock-based award. We estimated the weighted-average expected life based on the contractual and vesting terms of the stock options, as well as historical cancellation and exercise data.
Summary of Stock Option Activity
The table below presents a summary of stock option activity under our equity incentive plans (in thousands, except for price per share and contractual life information):
 
Number
of
Shares
 
Weighted-
Average
Exercise
Price
per Share
 
Weighted-
Average
Remaining
Contractual
Life
(in Years)
 
Aggregate
Intrinsic
Value(1)
Outstanding at December 31, 2018
15,930

 
$
26.18

 
 
 
 
Options granted
1,842

 
21.86

 
 
 
 
Options exercised
(1,629
)
 
11.68

 
 
 
 
Options forfeited & canceled
(1,258
)
 
49.91

 
 
 
 
Outstanding at December 31, 2019
14,885

 
$
25.23

 
4.31
 
$
73,134

 
 
 
 
 
 
 
 
Exercisable at December 31, 2019
9,938

 
20.79

 
3.14
 
$
66,707

_______________________________________________________________
(1)
Aggregate intrinsic value represents the difference between the exercise price of the option and the closing market price of our common stock on December 31, 2019.
The intrinsic value of options exercised during the years ended December 31, 2019, 2018 and 2017 totaled $30.6 million, $303.4 million and $84.0 million, respectively.
Summary of RSU Activity
A summary of RSU award activity is as follows (in thousands except for per share amounts):
 
Units Issued
 
Weighted-
Average
Grant
Date Fair
Value
 
Aggregate
Intrinsic
Value(1)
Balance at December 31, 2018
3,320

 
$
41.57

 
 
Granted
3,189

 
23.32

 
 
Vested and released
(1,159
)
 
36.33

 
 
Forfeited and canceled
(415
)
 
43.19

 
 
Balance at December 31, 2019
4,935

 
$
30.85

 
$
106,506


The fair value of restricted stock that vested in the years ended December 31, 2019, 2018 and 2017 totaled $32.4 million, $80.4 million and $22.3 million, respectively.