Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 01, 2019
Document Information [Line Items]
Entity Registrant Name	NextDecade Corp.
Entity Central Index Key	0001612720
Trading Symbol	next
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Current Reporting Status	Yes
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	true
Entity Common Stock, Shares Outstanding (in shares)		120,762,858
Entity Shell Company	false
Document Type	10-Q
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false
Title of 12(b) Security	Common Stock, $0.0001 par value