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Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense (Tables)
 12 Months Ended
Dec. 31, 2019
Notes Tables  
Share-based Payment Arrangement, Option, 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, 201
6
    
721,657
      
1,116,472
     $
9.56
      
3.70
     $
8.70
  
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
    
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
  
Additional shares reserved
(2)
    
700,000
      
 
      
 
      
 
      
 
  
Options exercised
    
-
      
(160,798)
     $
10.03
      
-
      
-
  
Options cancelled (expired or forfeited)
    
51,208
      
(51,208)
      
24.61
      
-
      
-
  
Stock awards granted
    
(1,538,128)
      
-
      
-
      
-
      
-
  
Stock awards cancelled (expired or forfeited)
    
407,320
      
-
      
-
      
-
      
-
  
Balances as of December 31, 2019
    
761,705
      
295,699
     $
25.52
      
3.19
     $
3.04
  
Exercisable as of December 31, 2019
    
 
      
222,400
     $
22.16
      
2.83
     $
3.04
  
Vested and expected to vest, net of estimated forfeitures, as of December 31, 2019
    
 
      
285,462
     $
25.08
      
3.00
     $
3.06
  
Share-based Payment Arrangement, Option, Exercise Price Range [Table Text Block]
Exercise
Prices
 
  
Number
Outstanding
    
Contractual
Life
(in
years)
    
Number
Exercisable
  
   $8.80
  
 
  
 
48,864
      
0.48
      
48,864
  
$9.65  –
$10.79
 
  
 
33,743
      
1.98
      
32,806
  
$10.80  –
$14.04
 
  
 
36,146
      
3.24
      
33,752
  
$15.32  –
$18.55
 
  
 
10,896
      
3.74
      
8,584
  
   $19.55
  
 
  
 
12,000
      
4.32
      
8,292
  
   $25.70
  
 
  
 
38,625
      
4.59
      
23,594
  
   $39.30
  
 
  
 
72,000
      
4.42
      
41,230
  
   $43.40
  
 
  
 
6,510
      
0.25
      
2,849
  
   $47.40
  
 
  
 
29,915
      
3.72
      
19,116
  
   $53.90
  
 
  
 
7,000
      
5.20
      
3,313
  
$
8.80
 
–
$
53.90
 
  
 
295,669
   
 
2.83
    
 
222,400
  
Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, 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, 2016
    
445,317
     $
11.15
      
 
 
   $
7,726
  
Granted
    
412,208
     $
28.74
      
 
 
    
 
  
Vested
(3)
    
(224,799
)
   $
10.91
     $
5,168
(4)
    
 
  
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
(5)
    
 
  
Forfeited
    
(69,905
)
   $
20.01
      
 
 
    
 
  
Outstanding at December 31, 2018
    
474,291
     $
38.44
      
 
 
   $
8,072
  
Granted
    
963,814
     $
18.68
      
 
 
    
 
  
Vested
(3)
    
(172,281
 )   $
33.66
     $
6,169
(6)
    
 
  
Forfeited
    
(161,022
 )   $
37.91
      
 
 
    
 
  
Outstanding at December 31, 2019
    
1,104,802
     $
22.10
      
 
 
   $
37,442
  
Share-based Payment Arrangement, Cost by Plan [Table Text Block]
   
Year Ended December 31,
  
   
201
9
    
201
8
    
201
7
  
Stock options
   $
622
     $
838
     $
815
  
RSUs
    
4,786
      
4,648
      
1,813
  
PSUs
    
3,948
      
1,105
      
2,093
  
ESPP
    
476
      
566
      
389
  
Total stock-based compensation expense
   $
9,832
     $
7,157
     $
5,110
  
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]
   
Year Ended December 31,
  
   
201
9
    
201
8
    
201
7
  
Cost of revenue
   $
1,572
     $
743
     $
660
  
Sales and marketing
    
4,510
      
2,105
      
1,642
  
Research and development
    
1,536
      
824
      
936
  
General and administrative
    
2,214
      
3,485
      
1,872
  
Total stock-based compensation expense
   $
9,832
     $
7,157
     $
5,110
  
Valuation Assumptions and Fair Value of Stock Options and ESPP Grants [Table Text Block]
   
Stock Options And ESPP
    
Stock Purchase Plan
  
   
201
9
    
201
8
    
201
7
    
201
9
    
201
8
    
201
7
  
                                                 
Expected term (in years)
(1)
    
3.65
      
3.70
      
3.70
      
0.50
      
0.50
      
0.50
  
Risk-free interest rate
(2)
    
1.64
%
    
2.60
%
    
1.73
%
    
2.49
%
    
2.34
%
    
1.14
%
Volatility
(3)
    
54
%
    
44
%
    
40
%
    
70
%
    
61
%
    
42
%
Dividend yield
(4)
    
-
%
    
-
%
    
-
%
    
-
%
    
-
%
    
-
%
Weighted average estimated fair value at grant date
   $
14.83
     $
18.00
     $
9.98
     $
9.60
     $
9.60
     $
8.21