Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Mar. 18, 2021	Jun. 30, 2020
Document Information [Line Items]
Entity Central Index Key	0001612720
Entity Registrant Name	NextDecade Corp.
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	FY
Document Fiscal Year Focus	2020
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	001-36842
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-5723951
Entity Address, Address Line One	1000 Louisiana Street, Suite 3900
Entity Address, City or Town	Houston
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	77002
City Area Code	713
Local Phone Number	574-1880
Title of 12(b) Security	Common stock $0.0001 par value
Trading Symbol	NEXT
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 66,700,000
Entity Common Stock, Shares Outstanding		122,174,938