Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Aug. 01, 2019
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, 2019
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Filer Category	Accelerated Filer
Common Stock Outstanding		110,535,922
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2