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Reinsurance - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Insurance [Abstract]
Loss reserve balance	$ 33.1	$ 8.7

X

- Definition

Loss Reserve, Net

+ References

No definition available.

+ Details

Name:	nni_LossReserveNet
Namespace Prefix:	nni_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- References

No definition available.

+ Details

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