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Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Share-based Compensation, Stock Options, Activity [Table Text Block]
   
 
 
 
  
Options Outstanding
  
   
Shares
Available
For Grant
    
Number of
Shares
    
Weighted-
Average
Exercise
Price
    
Weighted-Average
Remaining
Contractual Life
(in years)
    
Aggregate
Intrinsic
Value
(in $ millions
)
(1)
  
Balances as of December 31, 2015
    
1,263,425
      
2,148,797
     $
9.31
      
3.4
     $
7.9
  
Options granted
    
(162,000
)
    
162,000
     $
11.55
      
 
      
 
  
Options exercised
    
—
      
(1,051,138
)
   $
8.89
      
 
      
 
  
Options cancelled (expired or forfeited)
    
143,187
      
(143,187
)
   $
12.93
      
 
      
 
  
Stock awards granted
    
(1,018,005
)
    
—
      
—
      
 
      
 
  
Stock awards cancelled (expired or forfeited)
    
495,050
      
—
      
—
      
 
      
 
  
                                         
Balances as of December 31, 2016
    
721,657
      
1,116,472
     $
9.56
      
3.7
     $
8.7
  
                                         
Options granted
    
(278,250
)
    
278,250
     $
31.00
      
 
      
 
  
Options exercised
    
—
      
(488,398
 )   $
8.96
      
 
      
 
  
Options cancelled (expired or forfeited)
    
66,405
      
(66,405
 )   $
16.54
      
 
      
 
  
Stock awards granted
    
(873,881
)
    
—
      
—
      
 
      
 
  
Stock awards cancelled (expired or forfeited)
    
258,935
      
—
      
—
      
 
      
 
  
Additional shares reserved
(2)
    
1,600,000
      
—
      
—
      
 
      
 
  
Balances as of December 31, 2017
    
1,494,866
      
839,919
     $
16.46
      
3.99
     $
24.4
  
Options granted
    
(21,010
 )    
21,010
     $
50.65
      
 
      
 
  
Options exercised
    
—
      
(271,902
 )   $
9.99
      
 
      
 
  
Options cancelled (expired or forfeited)
    
81,322
      
(81,322
 )   $
21,55
      
 
      
 
  
Stock awards granted
    
(562,070
 )    
 
      
 
      
 
      
 
  
Stock awards cancelled (expired or forfeited)
    
148,197
      
 
      
 
      
 
      
 
  
Balances as of December 31, 2018
    
1,141,305
      
507,705
     $
20.52
      
3.52
     $
2.00
  
Exercisable as of December 31, 2018
    
 
      
335,348
     $
14.68
      
2.73
     $
1.87
  
Vested and expected to vest, net of estimated forfeitures, as of December 31, 2018
    
 
      
485,469
     $
19.88
      
3.42
     $
19.86
  
Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block]
Exercise Prices
      
Number
Outstanding
      
Contractual Life
(in years)
      
Number
Outstanding
  
  $6.88      
–
      
$7.44
      
28,832
      
0.58
      
28,832
  
         
$8.80
      
 
      
97,268
      
1.44
      
97,268
  
  $9.65      
–
      
$10.79
      
54,564
      
3.01
      
49,544
  
  $10.80      
–
      
$11.24
      
57,521
      
3.18
      
47,293
  
  $11.67      
–
      
$17.90
      
72,583
      
2.44
      
48,855
  
  $18.55      
–
      
$25.70
      
71,250
      
5.21
      
28,512
  
         
$39.30
      
 
      
72,000
      
5.83
      
23,230
  
         
$43.40
      
 
      
6,510
      
6.45
      
—
  
         
$47.40
      
 
      
38,927
      
5.86
      
10,564
  
         
$53.90
      
 
      
8,250
      
5.30
      
1,250
  
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]
   
Number
of
    
Weighted-Average
Grant-
Date
Fair
    
Aggregate
Fair
Value
(1)
    
Aggregate
Intrinsic Value

(2)
  
   
Shares
    
Value
    
(in thousands)
    
(in thousands)
  
Outstanding
at
December
31,
2015
    
371,630
     $
12.39
      
 
 
   $
4,753
  
Granted
    
480,191
     $
10.80
      
 
 
    
 
  
Vested
(3)
    
(172,990
 )   $
12.56
     $
1,906
 
(5)
    
 
  
Forfeited
    
(233,514
 )   $
11.36
      
 
 
    
 
  
Outstanding
at
December
31,
2016
    
445,317
     $
11.15
      
 
 
   $
7,726
  
Granted
    
412,208
     $
28.74
      
 
 
    
 
  
Vested
(3)
    
(224,799
 )   $
10.91
     $
5,168
 
(6)
    
 
  
Forfeited
    
(122,139
 )   $
13.56
      
 
 
    
 
  
Outstanding
at
December
31,
2017
    
510,587
     $
24.88
      
 
 
   $
23,155
  
Granted
    
265,124
     $
44.57
      
 
 
    
 
  
Vested
(3)
    
(231,515
 )   $
21.10
     $
9,483
 
(6)
    
 
  
Forfeited
    
(69,905
 )   $
20.01
      
 
 
    
 
  
Outstanding
at
December
31,
2018
    
474,291
     $
38.44
      
 
 
   $
8,072
  
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block]
   
Year Ended December 31,
  
   
201
8
    
201
7
    
201
6
  
Stock options
   $
838
     $
815
     $
989
  
RSUs
    
4,648
      
1,813
      
1,508
  
PSUs
    
1,105
      
2,093
      
967
  
ESPP
    
566
      
389
      
249
  
Total stock-based compensation expense
   $
7,157
     $
5,110
     $
3,713
  
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
   
Year Ended December 31,
  
   
201
8
    
201
7
    
201
6
  
Cost of revenue
   $
743
     $
660
     $
341
  
Sales and marketing
    
2,105
      
1,642
      
1,179
  
Research and development
    
824
      
936
      
596
  
General and administrative
    
3,485
      
1,872
      
1,597
  
Total stock-based compensation expense
   $
7,157
     $
5,110
     $
3,713
  
Valuation Assumptions and Fair Value of Stock Options and ESPP Grants [Table Text Block]
   
Stock Options
    
Stock Purchase Plan
  
   
201
8
    
201
7
    
201
6
    
201
8
    
201
7
    
201
6
  
                                                 
Expected term (in years)
(1)
    
3.7
      
3.70
      
3.83
      
0.50
      
0.50
      
0.50
  
Risk-free interest rate
(2)
    
2.6
 %    
1.73
%
    
1.09
%
    
2.34
 %    
1.14
%
    
0.46
 %
Volatility
(3)
    
44
 %    
40
%
    
40
%
    
61
 %    
42
%
    
39
 %
Dividend yield
(4)
    
—
%
    
—
%
    
—
%
    
—
%
    
—
%
    
—
 %
                                                 
Weighted average estimated fair value at grant date
   $
18.0
     $
9.98
     $
3.72
     $
9.6
     $
8.21
     $
3.22