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Subsequent Events (Details) $ in Millions | Apr. 22, 2019 USD ($)
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Subsequent Events | EPC bid process | Bechtel Oil, Gas, and Chemicals, Inc. and Fluor Corporation |
Subsequent Events |
Invitation to Bid Contract Costs | $ 4.0

X
- Definition The amount of expenses relating to invitation of bid contract. + References No definition available. + Details
Name: | next_BidContractCostsInvitation
Namespace Prefix: | next_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=next_EngineeringProcurementAndConstructionBidProcessMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CounterpartyNameAxis=next_BechtelOilGasAndChemicalsInc.AndFluorCorporationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |