Summary of Changes in Estimated Fair Value of Embedded Derivatives Liabilities Measured on Recurring Basis (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($)
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Balance as of December 31, 2017	$ 8,854
Net decrease in fair value included in interest and other income, net	(1,634)
Balance as of March 31, 2018	$ 7,220

Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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