Name:	next_DebtSecuritiesAvailableForSaleLineItems
Namespace Prefix:	next_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Represents maturity period of mutual funds.

No definition available.

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

Represents the percentage of holdings that are non-investment grade rated.

No definition available.

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

Represents the percentage of investment holdings that are grade rated.

No definition available.

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

Represents the term of mutual fund.

No definition available.

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

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563