Note 11 - Net Loss Per Share Attributable to Common Stockholders (Details Textual) - shares shares in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Securities Excluded from Computation of Earnings Per Share Amount, Performance Conditions (in shares)	9.4	8.3

Securities that have not yet satisfied the performance conditions that are excluded from potentially dilutive securities.

No definition available.

Name:	next_SecuritiesExcludedFromComputationOfEarningsPerShareAmountPerformanceConditions
Namespace Prefix:	next_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration