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Investments and Fair Value Measurements - Investment Category and Length (Details)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2020
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Jun. 30, 2020
USD ($)
company
Jun. 30, 2019
USD ($)
Dec. 31, 2019
USD ($)
company
Summary of fair value and gross unrealized losses related to available-for-sale securities [Abstract]            
Fair value, in loss position less than twelve months           $ 4,515
Gross unrealized loss, in loss position less than twelve months           (9)
Fair value, total in loss position           4,515
Gross unrealized loss, total in loss position           (9)
Minority Investments            
Investments, equity method $ 5,613     $ 5,613   $ 5,961
Number of equity method investments | company       5   5
Impairment charge 0 $ 0 $ 1,100 $ 0 $ 1,100  
Xilingol Tongli Germanium Co. Ltd Investment | Xilingol Tongli Germanium Co. Ltd Investment            
Minority Investments            
Investments, equity method $ 0     $ 0    
Ownership (as a percent) 25.00%     25.00%    
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           $ 4,515
Gross unrealized loss, in loss position less than twelve months           (9)
Fair value, total in loss position           4,515
Gross unrealized loss, total in loss position           (9)
Other assets            
Minority Investments            
Investments, equity method $ 5,600     $ 5,600   $ 6,000