XML 56 R42.htm IDEA: XBRL DOCUMENT
v3.19.1

Preferred Stock and Common Stock Warrants - Beneficial Conversion Feature (Details) - Series A Convertible Preferred stock - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Preferred Stock and Common Stock Warrants |  |
Deemed dividends - accretion of beneficial conversion feature - temporary equity | $ 551 | $ 0
BCF |  |
Preferred Stock and Common Stock Warrants |  |
Accretion of stock discount as a deemed dividend | $ 2,500 |

X
- Definition The amount of accretion of stock discount attributable to beneficial conversion feature as a deemed dividend. + References No definition available. + Details
Name: | next_AccretionOfDiscountAmount
Namespace Prefix: | next_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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
Name: | us-gaap_ClassOfStockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Value of accretion of temporary equity during the period due to unpaid dividends. + References No definition available. + Details
Name: | us-gaap_TemporaryEquityAccretionOfDividends
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=next_SeriesaConvertiblePreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ConversionOfStockByUniqueDescriptionAxis=next_BeneficialConversionFeatureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |