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Fair Value Measurements - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]      
Maturities of marketable securities $ 69,000,000 $ 18,000,000 $ 0
Gross realized gains from sales of marketable securities 17,467,000    
Gain on sale of marketable securities 4,000 0 0
Interest earned on marketable securities $ 6,000 $ 77,000 $ 0