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Income Taxes - Reconciliation (Details)
12 Months Ended
Dec. 31, 2017
Reconciliation of the federal statutory income tax rate  
U.S. federal statutory rate, beginning of year 35.00%
NextDecade LLC pre-merger net loss (5.00%)
Officers' compensation (12.00%)
Effect of 2017 Tax Act (7.00%)
Valuation allowance (11.00%)
Effective tax rate as reported 0.00%