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Fair Value of Financial Instruments - Narrative (Details) - USD ($)
Apr. 06, 2018
Mar. 27, 2018
Sep. 30, 2018
Debt Instrument [Line Items]      
Over-allotment option, term   13 days  
Over-allotment option   $ 22,500,000.0  
Additional proceeds $ 21,500,000    
Unsecured obligations | 2.50% Convertible notes due 2023      
Debt Instrument [Line Items]      
Aggregate principal amount 171,500,000 $ 150,000,000  
Interest rate   2.50%  
Proceeds received $ 171,500,000    
Fair value     $ 162,900,000