Note 9 - Debt (Details Textual) - Rio Grande [Member]	3 Months Ended
Sep. 30, 2023
CD Credit Facility and TCF Credit Facility [Member]
Debt Instrument, Covenant, Minimum Percentage of Projected Principal Amount Outstanding Be Hedged	75.00%
Debt Instrument, Covenant, Historical Debt Service Coverage Ratio, Minimum	1.1
Senior Secured Notes [Member]
Debt Instrument, Interest Rate, Stated Percentage	6.67%
Debt Instrument, Covenant, Historical Debt Service Coverage Ratio, Minimum	1.1
Debt Instrument, Covenant, Repurchase Price, Percentage	101.00%
Senior Secured Loans [Member]
Debt Instrument, Interest Rate, Stated Percentage	6.72%
Debt Instrument, Covenant, Historical Debt Service Coverage Ratio, Minimum	1.1
Debt Instrument, Covenant, Prepaid Price, Percent	101.00%

Requirement for minimum historical debt service coverage ratio under debt instrument.

No definition available.

Name:	next_DebtInstrumentCovenantHistoricalDebtServiceCoverageRatioMinimum
Namespace Prefix:	next_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents minimum percentage of projected principal amount outstanding to be hedged required by debt instrument.

+ References

No definition available.

+ Details

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

- Definition

Represents prepaid price to purchase debt as a percentage under debt instrument.

+ References

No definition available.

+ Details

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

- Definition

Represents repurchase price to repurchasing debt as a percentage under debt instrument.

+ References

No definition available.

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_ConsolidatedEntitiesAxis=next_RioGrandeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=next_CdCreditFacilityAndTcfCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=next_SeniorSecuredNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=next_SeniorSecuredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: