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Note 10 - Net Loss Per Share (Details Textual) - shares shares in Millions | 3 Months Ended | 6 Months Ended
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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

X
- Definition Securities that have not yet satisfied the performance conditions that are excluded from potentially dilutive securities. + References No definition available. + Details
Name: | next_SecuritiesExcludedFromComputationOfEarningsPerShareAmountPerformanceConditions
Namespace Prefix: | next_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration