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Cash, Cash Equivalents and Investments - Investment Category and Length (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
entity
Dec. 31, 2018
USD ($)
company
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
company
Dec. 31, 2016
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 $ 9,892,000 $ 9,892,000 $ 9,892,000 $ 29,527,000    
Gross unrealized loss, in loss position less than twelve months (32,000) (32,000) (32,000) (85,000)    
Fair value, in loss position greater than twelve months 12,935,000 12,935,000 12,935,000 2,342,000    
Gross unrealized loss, in loss position greater than twelve months (52,000) (52,000) (52,000) (8,000)    
Fair value, total in loss position 22,827,000 22,827,000 22,827,000 31,869,000    
Gross unrealized loss, total in loss position (84,000) (84,000) (84,000) (93,000)    
Minority Investments            
Investments, equity method $ 8,422,000 $ 8,422,000 8,422,000 9,849,000    
Equity method investments 7 7        
Impairment charge     0      
Cost method investments $ 0 $ 0 0 0    
One Gallium Company            
Minority Investments            
Investments, equity method           $ 0
Impairment charge       $ 313,000 $ 0  
Number of gallium companies with impairment charge | company       1    
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 717,000 717,000 717,000 $ 3,994,000    
Gross unrealized loss, in loss position less than twelve months (3,000) (3,000) (3,000) (16,000)    
Fair value, in loss position greater than twelve months 3,746,000 3,746,000 3,746,000 2,342,000    
Gross unrealized loss, in loss position greater than twelve months (24,000) (24,000) (24,000) (8,000)    
Fair value, total in loss position 4,463,000 4,463,000 4,463,000 6,336,000    
Gross unrealized loss, total in loss position (27,000) (27,000) (27,000) (24,000)    
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 9,175,000 9,175,000 9,175,000 25,533,000    
Gross unrealized loss, in loss position less than twelve months (29,000) (29,000) (29,000) (69,000)    
Fair value, in loss position greater than twelve months 9,189,000 9,189,000 9,189,000      
Gross unrealized loss, in loss position greater than twelve months (28,000) (28,000) (28,000)      
Fair value, total in loss position 18,364,000 18,364,000 18,364,000 25,533,000    
Gross unrealized loss, total in loss position (57,000) (57,000) (57,000) (69,000)    
Other assets            
Minority Investments            
Investments, equity method $ 8,400,000 $ 8,400,000 $ 8,400,000 $ 9,800,000