Derivative, Notional Amount Terminated

No definition available.

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- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

+ References

No definition available.

+ Details

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- 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

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- Definition

Fixed interest rate that will be received or paid upon exercise of the interest rate swaption contract.

+ References

No definition available.

+ Details

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- Definition

The increase (decrease) during the reporting period in the aggregate value of financial instruments used in operating activities, including trading securities, risk management instruments, and other short-term trading instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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- Definition

The cash outflow for a financial contract that meets the hedge criteria as either cash flow hedge, fair value hedge or hedge of net investment in foreign operations.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 27
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3581-108585

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from a financial contract that meets the hedge criteria as either cash flow hedge, fair value hedge or hedge of net investment in foreign operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 27
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3581-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3255-108585

+ Details

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- Details

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- Details

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- Details

Name:	nni_InterestRateSwapAxis=nni_FixedRateFloorIncomeMember
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- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_SwaptionMember
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- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateCapMember
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Derivative Financial Instruments - Narrative (Details) - USD ($)	1 Months Ended		3 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2015	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2014
1:3 basis swaps
Derivative [Line Items]
Weighted average basis point paid			0.097%		0.094%
Interest rate swap \| Interest rate swaps - floor income hedges
Derivative [Line Items]
Amounts received to terminate derivative instruments				$ 2,100,000
Amounts paid to terminate derivative instruments			$ 16,500,000
Notional amount, terminated			$ 700,000,000.0	$ 100,000,000.0
Interest rate swap options - floor income hedges
Derivative [Line Items]
Payments to enter into derivative instruments					$ 4,600,000	$ 9,100,000
Fixed interest rate of swap option			3.21%
Interest rate caps
Derivative [Line Items]
Payments to enter into derivative instruments	$ 300,000	$ 2,900,000