XML 45 R31.htm IDEA: XBRL DOCUMENT
v3.19.1

Investment Securities (Details) - JPMorgan Managed Income Fund - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Investment securities |  |
Maturity period of mutual funds | 1 year |
Duration of mutual fund | 6 months |
Non-investment grade rated holdings (as a percent) | 0.00% |
Fair Value | $ 56,088 | $ 72,453
Cost | $ 55,976 | $ 72,567
AAA rated |  |
Investment securities |  |
Investment holdings grade rated (as a percent) | 7.00% |

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

X
- Definition Represents maturity period of mutual funds. + References No definition available. + Details
Name: | next_MaturityPeriodOfMutualFunds
Namespace Prefix: | next_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of holdings that are non-investment grade rated. + References No definition available. + Details
Name: | next_PercentageOfHoldingsNonInvestmentGradeRated
Namespace Prefix: | next_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the percentage of investment holdings that are grade rated. + References No definition available. + Details
Name: | next_PercentageOfInvestmentHoldingsGradeRated
Namespace Prefix: | next_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the term of mutual fund. + References No definition available. + Details
Name: | next_TermOfMutualFund
Namespace Prefix: | next_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
Name: | us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of debt securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -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 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -Subparagraph (b)-(g) -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62557-112803 + Details
Name: | us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_InvestmentTypeAxis=next_JpmorganManagedIncomeFundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreditRatingFitchAxis=us-gaap_FitchAAARatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |