| **Fair Value Measurements - Level 3 Reconciliation (Details) (Guarantees [Member], USD $)In Millions, unless otherwise specified** | 3 Months Ended                                                                                                                                                                    | 6 Months Ended                                                                                                                                                                    |
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| Guarantees [Member]                                                                                                                |                                                                                                                                                                              |                                                                                                                                                                              |   |   |
| **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                | $ (8)nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityBeginningValue1/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember | $ (6)nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityBeginningValue1/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember | $ (8)nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityBeginningValue1/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember | $ (6)nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityBeginningValue1/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember |
| Transfers out of Level 3                                                                                                           | 0nav_FairValueMeasurementWithUnobservableInputsReconciliationTransfersOutOfLevel3/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember                            | 0nav_FairValueMeasurementWithUnobservableInputsReconciliationTransfersOutOfLevel3/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember                            | 0nav_FairValueMeasurementWithUnobservableInputsReconciliationTransfersOutOfLevel3/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember | 0nav_FairValueMeasurementWithUnobservableInputsReconciliationTransfersOutOfLevel3/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember |
| Issuances                                                                                                                          | 0nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisIssuances/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember                         | 0nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisIssuances/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember                         | 0nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisIssuances/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember | 0nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisIssuances/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember |
| Settlements                                                                                                                        | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember              | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember              | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Ending Value                   | (7)nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityEndingValue/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember       | (6)nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityEndingValue/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember       | (7)nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityEndingValue/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember | (6)nav_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetLiabilityEndingValue/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember |
| Change in unrealized gains on assets and liabilities still held                                                                    | $ 0nav_FairValueChangeInUnrealizedGainsOnAssetsAndLiabilitiesStillHeld1/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember                                      | $ 0nav_FairValueChangeInUnrealizedGainsOnAssetsAndLiabilitiesStillHeld1/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember                                      | $ 0nav_FairValueChangeInUnrealizedGainsOnAssetsAndLiabilitiesStillHeld1/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember | $ 0nav_FairValueChangeInUnrealizedGainsOnAssetsAndLiabilitiesStillHeld1/ nav_FairValueByAssetAndLiabilityClassAxis= us-gaap_GuaranteesMember |

| X                                                                                                                                                                                                                        |
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| X                                                                                                                                                                                                                                                                                                                                 |
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| - Definition
Fair Value Change in Unrealized Gains on Assets and Liabilities Still Held1
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                           |
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| - Definition
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+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                   |
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| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Ending Value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Issuances
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Transfers out of Level 3
+ References
No definition available.
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| 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=36462937&loc=d3e19207-110258
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