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Preferred Stock and Common Stock Warrants - Warrants (Details) $ in Thousands | 3 Months Ended
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Mar. 31, 2019 USD ($)
Preferred Stock and Common Stock Warrants |
Loss recognized on revalued warrants | $ 197

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

X
- References No definition available. + Details
Name: | us-gaap_EquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration