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Segment Reporting - Narrative (Details)	12 Months Ended
	Dec. 31, 2024
Segment Reporting [Abstract]
Income tax allocation to segments, percent	24.00%

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- Definition

Effective Income Tax Rate Reconciliation, Percent, Allocated To Segments

+ References

No definition available.

+ Details

Name:	nni_EffectiveIncomeTaxRateReconciliationPercentAllocatedToSegments
Namespace Prefix:	nni_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

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- References

No definition available.

+ Details

Name:	us-gaap_SegmentReportingAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration