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Income Taxes - Effective Income Tax Rate (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of the federal statutory income tax rate    
U.S. federal statutory rate, beginning of year 21.00% 35.00%
NextDecade LLC pre-merger net loss   (5.00%)
Officers' compensation (5.00%) (12.00%)
U.S. tax reform rate change   (7.00%)
Other (1.00%)  
Valuation allowance (15.00%) (11.00%)
Effective tax rate as reported 0.00% 0.00%