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Accrued Liabilities and Other Current Liabilities - Valve installation (Details) - Intrastate Pipeline Company - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended
--- | --- | ---
Apr. 30, 2018 | Mar. 31, 2019
Other Commitments [Line Items] |  |
Period after valve installation for reimbursement of costs | 30 days |
Percentage of valve costs for reimbursement | 50.00% |
Maximum reimbursement payment per agreement | $ 2.0 |
Valve reimbursement payment made during period |  | $ 2.0

X
- Definition Represents maximum amount of reimbursement payment by the entity in connection with the valve costs. + References No definition available. + Details
Name: | next_MaximumReimbursementPaymentPerAgreement
Namespace Prefix: | next_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Percentage of costs incurred in connection with the valve for reimbursement to be paid by the entity. + References No definition available. + Details
Name: | next_PercentageOfValveCostsForReimbursement
Namespace Prefix: | next_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the period after valve installation, in which the entity reimburses costs per agreement. + References No definition available. + Details
Name: | next_PeriodForReimbursementAfterValveInstallation
Namespace Prefix: | next_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Cash outflow for valve reimbursement payment during the period. + References No definition available. + Details
Name: | next_ValveReimbursementPayment
Namespace Prefix: | next_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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_CounterpartyNameAxis=next_IntrastatePipelineCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |