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Note 11 - Stockholders' Equity
 6 Months Ended
Jun. 30, 2018
Notes to Financial Statements  
Stockholders' Equity Note Disclosure [Text Block]
11.
Stockholders’ Equity
 
Common Stock Offering
s
 
In
January 2017,
we sold
14.1
million shares of our common stock at a public offering price of
$12.20
per share. We received total net proceeds of
$163.7
million (net of
$8.8
million in issuance costs) from the sale of the common stock.
 
In
December 2017,
we sold
9.7
million shares of our common stock at a public offering price of
$15.50
per share. We received total net proceeds of
$142.6
million (net of
$7.4
million in issuance costs) from the sale of the common stock.
 
In
April 2018,
we sold
6.9
million shares of our common stock at a public offering price of
$26.00
per share. We received total net cash proceeds of
$168.7
million (net of
$10.7
million in issuance costs) from the sale of the common stock.
 
In
June 2018,
we sold
5.1
million shares of our common stock at a public offering price of
$39.50
per share. We received total net cash proceeds of
$194.4
million (net of
$5.6
million in issuance costs) from the sale of the common stock.
 
Stock Option
Activity
 
The following table summarizes the stock option activity for the
six
months ended
June 30, 
2018:
 
   
 
 
 
  
 
 
 
  
Weighted-
  
   
 
 
 
  
 
 
 
  
a
verage
  
   
 
 
 
  
Weighted-
    
Remaining
  
   
 
 
 
  
a
verage
    
Contractual
  
   
Shares
    
Exercise
    
Term
  
   
(in thousands)
    
Price
    
(Years)
  
Balance at December 31, 2017
    
13,463
     $
15.03
      
8.01
  
Granted
    
355
     $
27.61
      
 
  
Exercised
    
(938
)
   $
11.34
      
 
  
Expired and forfeited
    
(366
)
   $
18.05
      
 
  
Balance at June 30, 2018
    
12,514
     $
15.58
      
7.73
  
 
For the
six
months ended
June 30, 2018,
938,000
shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of
$10.6
million.
 
Stock-based Compensation
 
The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):
 
   
 
Three Months Ended
June 30,
    
 
Six
Months Ended
June 30,
  
   
2018
    
2017
    
2018
    
2017
  
                                 
Research and development
   $
3,115
     $
2,955
     $
6,151
     $
6,163
  
General and administrative
    
2,308
      
2,552
      
4,584
      
4,926
  
Sales and marketing
    
2,398
      
2,643
      
4,787
      
5,082
  
Total stock-based compensation expense
   $
7,821
     $
8,150
     $
15,522
     $
16,171
  
 
As of
June 30, 2018,
there was
$75.3
million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of
2.6
 years.
 
We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:
 
   
June 30
,
  
   
20
18
    
20
17
  
Risk-free interest rate
    
2.7
 %    
2.0
 %
Dividend yield
    
0.0
 %    
0.0
 %
Volatility
    
71.1
 %    
81.4
 %
Expected life (years)
    
6
      
6
  
 
We estimated the fair value of each purchase right granted under our
1997
Employee Stock Purchase Plan at the beginning of each new offering period using the Black-Scholes option pricing model with the following weighted-average assumptions:
 
   
June 30,
  
   
20
18
    
20
17
  
Risk-free interest rate
    
2.1
 %    
1.0
 %
Dividend yield
    
0.0
 %    
0.0
 %
Volatility
    
59.8
 %    
56.0
 %
Expected life (years)
    
6
      
6