Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurements, Level 3 | 3.25% Convertible Senior Notes (due 2020)
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	$ 23,490	$ 22,469
Cash Equivalents | Fair Value Measurements, Level 1
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Assets measured at fair value on recurring basis	8,169	13,718
Carrying Amount | 3.25% Convertible Senior Notes (due 2020)
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Convertible senior notes	23,490	22,469
Carrying Amount | Cash Equivalents
Schedule Of Servicing Assets At Fair Value And Amortized Value [Line Items]
Assets measured at fair value on recurring basis	$ 8,169	$ 13,718