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Segment Reporting - Core Earnings Adjustments to GAAP (Parenthetical) (Detail)	6 Months Ended
	Jun. 30, 2021 USD ($)
Segment Reporting [Abstract]
Amount that will be equal to mark-to-market gain or loss over the life of the contract	$ 0

X

- Definition

Mark-to-market gain or loss over life of contract.

+ References

No definition available.

+ Details

Name:	navi_MarkToMarketGainOrLossOverLifeOfContract
Namespace Prefix:	navi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

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