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Note 1 - Background and Basis of Presentation (Details Textual)	Sep. 30, 2020 a
Texas City, Texas [Member]
Area of Real Estate Property (Acre)	994

X

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	srt_StatementGeographicalAxis=next_TexasCityTexasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: