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

Represents maximum amount of reimbursement payment by the entity in connection with the valve costs.

No definition available.

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

- 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

- 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

- 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

- 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

- Details

Name:	srt_CounterpartyNameAxis=next_IntrastatePipelineCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: