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Stock Plans - Stock-based compensation (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Jul. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based compensation expense        
Total stock-based compensation expense before taxes $ 2,000 $ 22,495 $ 34,178 $ 31,404
Tax benefit   3,921 11,108 10,130
Stock-based compensation cost related to non-vested equity awards not yet recognized as an expense   $ 54,200    
Weighted-average period over which stock-based compensation cost related to non-vested equity awards is expected to be recognized   2 years 6 months    
Severance costs   $ 1,100    
ESPP Plan        
Share-based compensation expense        
Total stock-based compensation expense before taxes   382 817 624
Stock options        
Share-based compensation expense        
Total stock-based compensation expense before taxes   6,311 7,778 8,495
Restricted stock awards        
Share-based compensation expense        
Total stock-based compensation expense before taxes   15,802 25,583 22,285
Cost of revenues        
Share-based compensation expense        
Total stock-based compensation expense before taxes   4,602 7,310 6,922
Research and development        
Share-based compensation expense        
Total stock-based compensation expense before taxes   6,030 8,891 7,461
Selling, general and administrative        
Share-based compensation expense        
Total stock-based compensation expense before taxes   $ 11,863 $ 17,977 $ 17,021