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Cash, Cash Equivalents and Investments - Investment Category and Length (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
entity
company
Dec. 31, 2017
USD ($)
company
Dec. 31, 2017
USD ($)
company
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Mar. 31, 2017
USD ($)
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]            
Fair value, in loss position less than twelve months $ 29,527,000 $ 29,527,000 $ 29,527,000 $ 10,248,000    
Gross unrealized loss, in loss position less than twelve months     (85,000) (55,000)    
Fair value, in loss position greater than twelve months 2,342,000 2,342,000 2,342,000 4,595,000    
Gross unrealized loss, in loss position greater than twelve months     (8,000) (12,000)    
Fair value, total in loss position 31,869,000 31,869,000 31,869,000 14,843,000    
Gross unrealized loss, total in loss position     (93,000) (67,000)    
Minority Investments            
Investments in privately-held companies $ 5,578,000 $ 5,578,000 5,578,000 6,599,000    
Equity method investments 7 7        
Cost method investments $ 0 $ 0 $ 0 0    
One Gallium Company            
Minority Investments            
Investments in privately-held companies           $ 0
Number of gallium companies with impairment charge | company 1 1 1      
Impairment charge     $ 313,000 0 $ 0  
Certificates of Deposit [Member]            
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]            
Fair value, in loss position less than twelve months $ 3,994,000 $ 3,994,000 3,994,000 5,211,000    
Gross unrealized loss, in loss position less than twelve months     (16,000) (20,000)    
Fair value, in loss position greater than twelve months 2,342,000 2,342,000 2,342,000 1,200,000    
Gross unrealized loss, in loss position greater than twelve months     (8,000)      
Fair value, total in loss position 6,336,000 6,336,000 6,336,000 6,411,000    
Gross unrealized loss, total in loss position     (24,000) (20,000)    
Corporate Bonds [Member]            
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]            
Fair value, in loss position less than twelve months 25,533,000 25,533,000 25,533,000 5,037,000    
Gross unrealized loss, in loss position less than twelve months     (69,000) (35,000)    
Fair value, in loss position greater than twelve months       3,395,000    
Gross unrealized loss, in loss position greater than twelve months       (12,000)    
Fair value, total in loss position 25,533,000 25,533,000 25,533,000 8,432,000    
Gross unrealized loss, total in loss position     (69,000) (47,000)    
Other assets            
Minority Investments            
Investments in privately-held companies $ 9,800,000 $ 9,800,000 $ 9,800,000 $ 11,300,000