Name:	next_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfAdditionalSharesIssued
Namespace Prefix:	next_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

This element represents the amount released from escrow from a business combination, which will be used for development activities.

No definition available.

Name:	next_BusinessCombinationEscrowReleasedToFundDevelopmentActivities
Namespace Prefix:	next_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the milestone achievement minimum volume of liquefied natural gas (LNG) per annum for additional shares to be granted as a result of a business combination.

No definition available.

Name:	next_BusinessCombinationMilestoneMinimumVolumeOfLiquefiedNaturalGasPerAnnum
Namespace Prefix:	next_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	duration

Number of shares of equity interests issued or issuable to acquire entity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Name:	us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Percentage of voting equity interests acquired at the acquisition date in the business combination.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463