Net Impact of Changes in Fair Value of Call Option and Embedded Cash Conversion Option - (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments Gain Loss [Line Items]
Net loss included in other income, net	$ 375	$ 163
1.25% Call Option [Member]
Derivative Instruments Gain Loss [Line Items]
Net loss included in other income, net	(6,074)	(21,168)
1.25% Notes Embedded Cash Conversion Option [Member]
Derivative Instruments Gain Loss [Line Items]
Net loss included in other income, net	$ 6,449	$ 21,331

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Name:	us-gaap_UnrealizedGainLossOnDerivatives
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration