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Stockholders' equity (Tables)
 12 Months Ended
Dec. 31, 2017
Stockholders' equity [Abstract]  
Option award activity
The following is a summary of stock option award activity under the 2006 Plan:

  
2006 Plan
    
  
Number of Shares
  
Weighted-Average Exercise Price
  
Aggregate Intrinsic Value
 
Outstanding at December 31, 2016
  
2,559,331
  
$
22.94
  
$
25,348,245
 
Granted
  
427,821
   
31.13
     
Exercised
  
(792,795
)
  
19.95
     
Forfeited
  
(72,952
)
  
29.30
     
Outstanding at December 31, 2017
  
2,121,405
  
$
25.48
  
$
44,518,585
 
Exercisable at December 31, 2017
  
1,307,330
  
$
22.63
  
$
31,170,967
 
Options expected to vest at December 31, 2017
  
632,954
  
$
29.91
  
$
10,480,716
 
Restricted stock units activity
The following is a summary of restricted stock unit award activity under the 2006 Plan:

  
Number of Shares
  
Weighted-Average Grant Price
  
Aggregate Intrinsic Value
 
Outstanding at December 31, 2016
  
875,584
  
$
28.94
  
$
28,754,179
 
Granted
  
480,959
   
31.49
     
Vested
  
(423,840
)
  
30.52
     
Forfeited
  
(80,983
)
  
29.21
     
Outstanding at December 31, 2017
  
851,720
  
$
30.84
  
$
39,579,428
 
Allocated stock-based compensation expense
Stock-based compensation expense was recorded in the following financial statement line items:

  
Year Ended December 31,
 
(in thousands)
 
2017
  
2016
  
2015
 
Cost of product sales
 
$
1,076
  
$
997
  
$
1,183
 
Research and development
  
2,526
   
2,297
   
2,324
 
Selling, general and administrative
  
11,611
   
14,062
   
11,234
 
Continuing operations
  
15,213
   
17,356
   
14,741
 
Discontinued operations
  
-
   
1,121
   
1,107
 
Total stock-based compensation expense
 
$
15,213
  
$
18,477
  
$
15,848