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Cash and Marketable Securities (Narrative) (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Amortized cost of securities         $ 7,000,000
Fair value of securities         $ 7,000,000
Marketable securities, realized gain (loss)     $ (3,000,000)    
Unrealized losses on marketable securities $ 0 $ 0   $ 0  
Gross unrealized gains of securities 0 0 0 0  
Gross unrealized losses of securities 0 0 0 0  
Proceeds from sale of available-for-sale securities 0 $ 0 0 0  
Maturities of available-for-sale securities     $ 9,000,000 $ 9,000,000  
Maximum [Member]          
Marketable securities, maturity term     1 year    
Amortized cost of securities 1,000,000   $ 1,000,000    
Fair value of securities $ 1,000,000   $ 1,000,000