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Allowance for Loan Losses - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage of loans granted forbearance that migrated to TDR classification	73.00%	72.00%
Payment default period for TDRs	60 days past due
Interest Rate Reduction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
TDR loans, Unpaid Principal Balance	$ 802	$ 948

X

- Definition

Payment default period for troubled debt restructurings.

+ References

No definition available.

+ Details

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Namespace Prefix:	navi_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of loans granted forbearance that migrated to troubled debt restructuring classification.

+ References

No definition available.

+ Details

Name:	navi_PercentageOfLoansGrantedForbearanceThatMigratedToTroubledDebtRestructuringClassification
Namespace Prefix:	navi_
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Balance Type:	na
Period Type:	duration

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

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

X

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5212-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: