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v3.19.1

Commitments and Contingencies (Details) $ in Millions | Mar. 31, 2019 USD ($)
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Enterprise Resource Planning |
Commitments and Contingencies |
Future commitments to be incurred | $ 0.9

X
- 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

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_OtherCommitmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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