The period of service agreements for a business sale.

No definition available.

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Namespace Prefix:	next_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The ownership percent sold of an equity method investment.

+ References

No definition available.

+ Details

Name:	next_EquityMethodInvestmentOwnershipPercentageSold
Namespace Prefix:	next_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the entity's equity method investment which has been sold.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8813-108599

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_EquityMethodInvestments
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Minimum amount of other commitment maturing in the next fiscal year following the latest fiscal year for commitments not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in this taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherCommitmentDueInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow associated with the sale of equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromSaleOfEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=next_SpectraEnergyTransmissionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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- Details

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Note 15 - Subsequent Events (Details Textual) - USD ($) $ in Millions	Mar. 02, 2020	Feb. 13, 2020	Mar. 01, 2020	Dec. 31, 2019
Enterprise Resource Planning [Member]
Other Commitment, Due in Next Twelve Months				$ 5.5
Spectra Energy Transmission [Member]
Equity Method Investments				$ 12.6
Subsequent Event [Member] \| Enterprise Resource Planning [Member]
Other Commitment, Due in Next Twelve Months	$ 4.0		$ 5.5
Subsequent Event [Member] \| Spectra Energy Transmission [Member]
Equity Method Investment, Ownership Percentage Sold		100.00%
Equity Method Investment, Amount Sold		$ 17.4
Proceeds from Sale of Equity Method Investments	$ 15.0	$ 15.0
Business Sale, Service Agreement Period	20 years	20 years
Subsequent Event [Member] \| Spectra Energy Transmission [Member] \| Maximum [Member]
Equity Method Investment, Amount Sold		$ 25.0