Note 12 - Commitments and Contingencies (Details Textual) - USD ($) shares in Millions, $ in Millions	1 Months Ended	9 Months Ended
Nov. 04, 2019	Sep. 30, 2019
Engineering, Procurement and Construction Contract [Member]
Other Commitment, Total		$ 30.3
Payments for Commitments		5.0
Commitment and Contingencies Value of Stock for Settlement		15.0
Engineering, Procurement and Construction Contract [Member] \| Subsequent Event [Member]
Stock Issued During Period, Shares, Contract Settlement	2.1
Enterprise Resource Planning [Member]
Other Commitments, Future Minimum Payments, Remainder of Fiscal Year		3.4
Other Commitment, Due in Second Year		$ 5.5

Value of common stock that can be issued to settle commitment.

No definition available.

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

- Definition

The amount of cash outflow for payments for commitments.

+ References

No definition available.

+ Details

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

- Definition

The number of shares issued during the period for contract settlement.

+ References

No definition available.

+ Details

Name:	next_StockIssuedDuringPeriodSharesContractSettlement
Namespace Prefix:	next_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherCommitment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Minimum amount of other commitment maturing in the second fiscal year following the latest fiscal year for commitments not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in this taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherCommitmentDueInSecondYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of other commitments maturing in the remainder of the fiscal year following the latest fiscal year ended.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherCommitmentsFutureMinimumPaymentsRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_OtherCommitmentsAxis=next_EngineeringProcurementAndConstructionContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_OtherCommitmentsAxis=next_EnterpriseResourcePlanningMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: