Net Impact of Changes in Fair Value of Call Option and Embedded Cash Conversion Option - (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments Gain Loss [Line Items]
Net (loss) income included in other income, net	$ 179	$ (460)	$ 93	$ (238)
1.25% Call Option [Member]
Derivative Instruments Gain Loss [Line Items]
Net (loss) income included in other income, net	(3,567)	15,043	(6,262)	(13,108)
1.25% Notes Embedded Cash Conversion Option [Member]
Derivative Instruments Gain Loss [Line Items]
Net (loss) income included in other income, net	$ 3,746	$ (15,503)	$ 6,355	$ 12,870