Summary of Changes in Estimated Fair Value of Embedded Derivative Liabilities Measured on Recurring Basis (Detail) $ in Thousands	12 Months Ended
Dec. 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	(6,357)
Balance as of December 31, 2018	$ 2,497

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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