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

The amount of gain (loss) recognized on revaluation of warrants.

No definition available.

Name:	next_GainLossOnRevaluationOfWarrants
Namespace Prefix:	next_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration