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Cash, Cash Equivalents and Short-Term Investments (Details) - USD ($)
Jun. 30, 2021
Dec. 31, 2020
Cash and Cash Equivalents    
Cash $ 29,609,000 $ 25,053,000
Cash equivalents:    
Money market mutual fund 30,748,000 11,820,000
Total cash and cash equivalents 60,357,000 36,873,000
Short-term investments:    
Total short-term investments 289,056,000 56,444,000
Cash, cash equivalents and short-term investments 349,413,000 93,317,000
U.S. Treasury notes and bills [Member]    
Short-term investments:    
Short-term Investments 229,131 23,309
Mutual Funds [Member]    
Short-term investments:    
Short-term Investments $ 59,925 $ 33,135