Note 3 - Sale of Equity Interests in Rio Bravo (Details Textual) - Rio Bravo [Member] - USD ($) $ in Millions	Mar. 02, 2020	Feb. 13, 2020	Jun. 30, 2022
Equity Method Investment, Ownership Percentage		100.00%
Disposal Group, Including Discontinued Operation, Consideration	$ 19.4
Proceeds from Divestiture of Businesses	$ 15.0
Equity Method Investments, Rights to The Natural Gas Firm Transportation Capacity Term (Year)		20 years
Sale of Business, Failure to Issue Notice, Maximum Right Price to Sell Back Equity Interests			$ 23.0

Represents percentage of business ownership sold.

No definition available.

Name:	next_BusinessOwnershipPercentageSold
Namespace Prefix:	next_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents term of rights to the natural gas firm transportation capacity.

+ References

No definition available.

+ Details

Name:	next_RightsToTheNaturalGasFirmTransportationCapacityTerm
Namespace Prefix:	next_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The maximum right price to sell back equity interests after the failure to issue a notice in the sale of a business.

+ References

No definition available.

+ Details

Name:	next_SaleOfBusinessFailureToIssueNoticeMaximumRightPriceToSellBackEquityInterests
Namespace Prefix:	next_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of consideration received or receivable for the disposal of assets and liabilities, including discontinued operation.

+ References

No definition available.

+ Details

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromDivestitureOfBusinesses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=next_RioBravoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: