Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Aug. 06, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	NextDecade Corp.
Entity Central Index Key	0001612720
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Common Stock Outstanding		108,851,167
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2