| **Fair Value Measurements - Level 3 Reconciliation (Details) - Guarantees [Member] - USD ($) $ in Millions**        | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Fair Value Assets And Liablities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]**     |           |           |   |   |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Beginning Value | $ (10)         | $ (8)          | $ (10) | $ (8) |
| Transfers out of Level 3                                                                                            | 0              | 0              | 0 | 0 |
| Issuances                                                                                                           | (10)           | 0              | (11) | 0 |
| Settlements                                                                                                         | 1              | 1              | 2 | 1 |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Ending Value    | (19)           | (7)            | (19) | (7) |
| Change in unrealized gains on assets and liabilities still held                                                     | $ 0            | $ 0            | $ 0 | $ 0 |

| X                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Assets And Liablities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
+ References
No definition available.
+ Details**Name:**nav_FairValueAssetsAndLiablitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems**Namespace Prefix:**nav_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Change in Unrealized Gains on Assets and Liabilities Still Held1
+ References
No definition available.
+ Details**Name:**nav_FairValueChangeInUnrealizedGainsOnAssetsAndLiabilitiesStillHeld1**Namespace Prefix:**nav_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Beginning Value1
+ References
No definition available.
+ Details**Name:**nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityBeginningValue1**Namespace Prefix:**nav_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Ending Value
+ References
No definition available.
+ Details**Name:**nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityEndingValue**Namespace Prefix:**nav_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Issuances
+ References
No definition available.
+ Details**Name:**nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisIssuances**Namespace Prefix:**nav_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Transfers out of Level 3
+ References
No definition available.
+ Details**Name:**nav_FairValueMeasurementWithUnobservableInputsReconciliationTransfersOutOfLevel3**Namespace Prefix:**nav_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258
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| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
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