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Fair Value Measurements - Additional Information (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Jul. 31, 2018
Significant Unobservable Inputs (Level 3)      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value of financial liabilities measured at fair value on a recurring basis $ 585,000 $ 585,000  
Changes in fair value of Level 3 liabilities $ 0    
3.5% Convertible Notes Due 2023      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt instrument, interest rate, stated percentage 3.50%   3.50%
Fair value of convertible debt $ 122,000,000 141,000,000  
Carrying value of convertible debt $ 150,314,000 $ 148,101,000 $ 143,723,000