Summary of Significant Accounting Policies - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Level 1 | 3.25% Convertible senior notes due in 2020
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	$ 0	$ 0
Level 2 | 3.25% Convertible senior notes due in 2020
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	0	0
Level 3 | 3.25% Convertible senior notes due in 2020
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	27,974	22,469
Cash Equivalents | Level 1
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Assets measured at fair value on recurring basis	0	13,718
Cash Equivalents | Level 2
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Assets measured at fair value on recurring basis	0	0
Cash Equivalents | Level 3
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Assets measured at fair value on recurring basis	0	0
Carrying Amount | 3.25% Convertible senior notes due in 2020
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	27,974	22,469	$ 0
Carrying Amount | Cash Equivalents
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Assets measured at fair value on recurring basis	$ 0	$ 13,718