Note 10 - Net Loss Per Share (Details Textual) - shares shares in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Securities Excluded from Computation of Earnings Per Share Amount, Performance Conditions (in shares)	6.2	2.6	6.2	2.6

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