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Financial Instruments - Amortized to fair value (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Apr. 01, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents $ 378,360 $ 341,966 $ 374,279 $ 301,320
Available-for-sale debt securities, gross unrealized gains 8 16    
Available-for-sale debt securities, gross unrealized losses (408) (270)    
Cash, cash equivalents, and available-for-sale securities, amortized cost 658,754 679,554    
Cash, cash equivalents, available-for-sale securities, fair value 658,354 679,300    
Total 279,994 337,334    
U.S. government agencies [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents   6,595    
Available-for-sale debt securities, amortized cost 75,075 79,722    
Available-for-sale debt securities, gross unrealized gains   1    
Available-for-sale debt securities, gross unrealized losses (153) (99)    
Cash, cash equivalents, available-for-sale securities, fair value 74,922 79,624    
Total 74,922 73,029    
Corporate debt securities [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents 91,026 27,277    
Available-for-sale debt securities, amortized cost 296,345 291,738    
Available-for-sale debt securities, gross unrealized gains 8 15    
Available-for-sale debt securities, gross unrealized losses (255) (171)    
Cash, cash equivalents, available-for-sale securities, fair value 296,098 291,582    
Total 205,072 264,305    
Cash [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents 117,904 115,028    
Money market funds [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents $ 169,430 $ 193,066