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Fair Value Measurements (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Investment [Line Items]      
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount $ 0 $ 0  
Private common equity securities 3,000    
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount 0 0  
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount 0 0  
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount 0 0  
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0 $ 9,500
Equity Securities [Member]      
Investment [Line Items]      
Trading Securities, Change in Unrealized Holding Gain (Loss) 0 0 330,400
Total debt securities      
Investment [Line Items]      
Trading Securities, Change in Unrealized Holding Gain (Loss) $ 10,000 $ (6,700) $ (12,400)