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Allowance for Loan Losses - Additional Information (Detail) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
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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

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- Definition Payment default period for troubled debt restructurings. + References No definition available. + Details
Name: | navi_PaymentDefaultPeriodForTroubledDebtRestructurings
Namespace Prefix: | navi_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of loans granted forbearance that migrated to troubled debt restructuring classification. + References No definition available. + Details
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Namespace Prefix: | navi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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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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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |