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Financial Instruments - Amortized to fair value (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jul. 01, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents $ 328,116 $ 341,966 $ 318,708 $ 301,320
Available-for-sale debt securities, gross unrealized gains 4 16    
Available-for-sale debt securities, gross unrealized losses (275) (270)    
Cash, cash equivalents, and available-for-sale securities, amortized cost 580,734 679,554    
Cash, cash equivalents, available-for-sale securities, fair value 580,463 679,300    
Total 252,347 337,334    
U.S. government agencies [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents 10,376 6,595    
Available-for-sale debt securities, amortized cost 75,032 79,722    
Available-for-sale debt securities, gross unrealized gains   1    
Available-for-sale debt securities, gross unrealized losses (135) (99)    
Cash, cash equivalents, available-for-sale securities, fair value 74,897 79,624    
Total 64,521 73,029    
Corporate debt securities [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents 29,980 27,277    
Available-for-sale debt securities, amortized cost 217,942 291,738    
Available-for-sale debt securities, gross unrealized gains 4 15    
Available-for-sale debt securities, gross unrealized losses (140) (171)    
Cash, cash equivalents, available-for-sale securities, fair value 217,806 291,582    
Total 187,826 264,305    
Cash [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents 134,519 115,028    
Money market funds [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents $ 153,241 $ 193,066