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Note 6 - Stockholders' Equity, Stock Plans and Stock-based Compensation Expense (Tables)
 12 Months Ended
Dec. 31, 2017
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, 2014
    
129,760
      
3,462,567
     $
9.39
      
3.4
     $
5.7
  
Additional shares reserved
(2)
    
1,300,000
      
—
      
 
      
 
      
 
  
Options granted
    
(129,000
)
    
129,000
     $
13.26
      
 
      
 
  
Options exercised
    
—
      
(1,141,904
)
   $
9.20
      
 
      
 
  
Options cancelled (expired or forfeited)
    
300,866
      
(300,866
)
   $
12.37
      
 
      
 
  
Stock awards granted
    
(430,580
)
    
—
      
—
      
 
      
 
  
Stock awards cancelled (expired or forfeited)
    
92,379
      
—
      
—
      
 
      
 
  
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, 201
6
    
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
(3)
    
1,600,000
      
—
      
—
      
 
      
 
  
Balances as of December 31, 2017
    
1,494,866
      
839,919
     $
16.46
      
3.99
     $
24.4
  
Exercisable as of December 31, 2017
    
 
      
467,794
     $
9.53
      
2.74
     $
16.8
  
Vested and expected to vest, net of estimated forfeitures, as of December 31, 2017
    
 
      
789,779
     $
15.61
      
3.86
     $
23.6
  
Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block]
 
    
 
    
Options
Outstanding
    
Options Exercisable
  
Range of
 
Exercise Prices
    
Number
Outstanding
    
Weighted-Average
Remaining
Contractual Life
(in years)
    
Number
Outstanding
    
Weighted-Average
Exercise
Price
  
 
$6.88
–
$7.15
    
84,739
    
1.43
    
84,739
     $
6.92
  
 
$7.44
–
$8.72
    
15,978
    
0.87
    
15,978
      
8.30
  
 
 
$8.80
 
    
141,956
    
2.45
    
141,956
      
8.80
  
 
$9.28
–
$9.97
    
95,535
    
3.24
    
73,402
      
9.69
  
 
$10.03
–
$10.86
    
88,044
    
4.46
    
51,517
      
10.38
  
 
$10.90
–
$11.67
    
91,021
    
4.22
    
50,356
      
11.36
  
 
$11.98
-
$17.90
    
109,896
    
4.66
    
49,033
      
13.36
  
 
$18.55
–
$25.70
    
88,500
    
5.87
    
813
      
18.55
  
 
 
$39.30
 
    
77,000
    
6.83
    
—
 		      
 —
 
  
 
 
$47.40
 
    
47,250
    
6.96
    
—
 		      
—
  
 
$6.88
–
$47.40
    
839,919
    
4.11 
    
467,794
     $
9.53
  
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]
   
Number
of
Shares
    
Weighted
-
Average
Grant-
Date Fair
Value
    
Aggregate
Fair Value
(1)
(in thousands)
 
  
Aggregate
Intrinsic Value
(2)
(in thousands)
  
Outstanding at December 31, 2014
    
434,321
     $
9.31
      
 
 
   $
4,639
  
Granted
    
203,104
     $
14.81
      
 
 
    
 
  
Vested
(3)
    
(222,220
 )   $
11.79
     $
3,285
(4)
    
 
  
Forfeited
    
(43,575
 )   $
9.09
      
 
 
    
 
  
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
  
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block]
   
Year Ended December 31,
  
   
2017
    
201
6
    
2015
  
Stock options
   $
815
     $
989
     $
1,438
  
RSUs
    
1,813
      
1,508
      
1,297
  
PSUs
    
2,093
      
967
      
1,167
  
ESPP
    
389
      
249
      
182
  
Total stock-based compensation expense
   $
5,110
     $
3,713
     $
4,084
  
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
   
Year Ended December 31,
  
   
2017
    
2016
    
2015
  
Cost of revenue
   $
660
     $
341
     $
447
  
Sales and marketing
                        
Employee
    
1,629
      
1,179
      
1,054
  
Non-employee
    
13
      
-
      
-
  
Research and development
    
936
      
596
      
662
  
General and administrative
    
1,872
      
1,597
      
1,921
  
Total stock-based compensation expense
   $
5,110
     $
3,713
     $
4,084
  
 
 
Valuation Assumptions and Fair Value of Stock Options and ESPP Grants
Valuation Assumptions and Fair Value of Stock Options and ESPP Grants [Table Text Block]
   
Stock Options
    
Stock Purchase Plan
  
   
2017
    
2016
    
2015
    
2017
    
2016
    
2015
  
Expected term (in years)
(1)
    
3.70
      
3.83
      
3.24
      
0.50
      
0.50
      
0.50
  
Risk-free interest rate
(2)
    
1.73
%
    
1.09
%
    
0.90
%
    
1.14
%
    
0.46
%
    
0.17
%
Volatility
(3)
    
40
%
    
40
%
    
30
%
    
42
%
    
39
%
    
36
%
Dividend yield
(4)
    
—
%
    
—
%
    
—
%
    
—
%
    
—
%
    
—
%
                                                 
Weighted average estimated fair value at grant date
   $
9.98
     $
3.72
     $
4.78
     $
8.21
     $
3.22
     $
3.51