Note 8 - Preferred Stock and Common Stock Warrants - Allocation of Net Cash Proceeds (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Proceeds from equity issuance	$ 24,500	$ 0
Series B Warrants [Member]
Net proceeds - Initial Fair Value Allocation	817
Per balance sheet upon issuance	817
Convertible Preferred Stock [Member]
Proceeds from equity issuance	24,500
Equity issuance costs	54
Net proceeds - Initial Fair Value Allocation	24,446
Series B Convertible Preferred Stock [Member]
Net proceeds - Initial Fair Value Allocation	23,629
Per balance sheet upon issuance	$ 23,629

The amount of cash inflow from issuance of convertible preferred stock and common stock warrants after deducting issuance costs and after beneficial conversion feature allocation.

No definition available.

+ Details

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

- Definition

The cash outflow for cost incurred directly with the issuance of an equity security.

+ References

No definition available.

+ Details

Name:	next_PaymentsOfEquityIssuanceCosts
Namespace Prefix:	next_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from issuance of convertible preferred stock and common stock warrants after deducting issuance costs.

+ References

No definition available.

+ Details

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

- Definition

The cash inflow from the issuance of common stock, preferred stock, treasury stock, stock options, and other types of equity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

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

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=next_SeriesBWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_ConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=next_SeriesBConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: