Note 8 - Accrued Liabilities and Other Current Liabilities (Details Textual) - Intrastate Pipeline Company [Member] - USD ($) $ in Millions	1 Months Ended	3 Months Ended
Apr. 30, 2018	Mar. 31, 2019
Percentage of Valve Costs for Reimbursement	50.00%
Maximum Reimbursement Payment Per Agreement	$ 2
Valve Reimbursement Payment		$ 2

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