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Fair Value Measurements - Additional Information (Details) - USD ($)
9 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Fair Value Disclosures [Abstract]    
Financial instruments transferred among fair value hierarchy levels $ 0  
Cash equivalents included of available-for-sale securities 11,800,000 $ 9,100,000
Short-term investments included of available-for-sale securities $ 196,170,000 $ 300,535,000