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Fair Value Measurements - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - 3.25% Convertible Senior Notes (due 2020) - Fair Value Measurements, Recurring - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Level 1    
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]    
Convertible senior notes $ 0 $ 0
Level 2    
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]    
Convertible senior notes 0 0
Level 3    
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]    
Convertible senior notes 30,397 27,974
Carrying Amount    
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]    
Convertible senior notes $ 30,397 $ 27,974