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Note 13 - Commitments and Contingencies (Details Textual) - BND [Member] $ in Millions | Apr. 30, 2020 USD ($) | Dec. 31, 2021 USD ($) | Mar. 06, 2019 a
--- | --- | --- | ---
Area Available for Lease (Acre) | a |  |  | 984
Lessee, Operating Lease, Liability, to be Paid Per Quarter | $ 1.5 |  |
Wetland Mitigation Measures [Member] |  |  |
Other Commitment, to be Paid, Year One |  | $ 0.5 |

X
- Definition Represents surface area available for lease. + References No definition available. + Details
Name: | next_AreaAvailableForLease
Namespace Prefix: | next_
Data Type: | dtr-types:areaItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of lease payments to be paid per quarter. + References No definition available. + Details
Name: | next_LesseeOperatingLeaseLiabilityToBePaidPerQuarter
Namespace Prefix: | next_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of commitment classified as other to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach). + References No definition available. + Details
Name: | us-gaap_OtherCommitmentDueInNextTwelveMonths
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | srt_CounterpartyNameAxis=next_BNDMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_OtherCommitmentsAxis=next_WetlandMitigationMeasuresMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |