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Common Stock Warrants - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Temporary Equity [Line Items] |  |
Loss on common stock warrant liabilities | $ 1,516 | $ 367
Common Stock Warrants |  |
Temporary Equity [Line Items] |  |
Loss on common stock warrant liabilities | $ 1,500 | $ 400

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- Definition The amount of gain (loss) recognized on revaluation of warrants. + References No definition available. + Details
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Namespace Prefix: | next_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_ClassOfWarrantOrRightAxis=next_CommonStockWarrantsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |