| **Postretirement Benefits - Defined Benefit Plans (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------|----------------|----------------|----------------|----------------|----------------|----------------|-----------------|
||
| **Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]**               |           |           |           |           |           |           |            |   |   |   |   |   |   |
| Contractual termination benefits                                                                             |           |           |           |           | $ 14           | $ 0            | $ 14            | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 |   |
| Amounts contributed to meet regulatory funding requirements                                                  |           |           |           |           | 33             | 29             | 98              | 86 |   |   |   |   |   |
| Defined Contribution Plan, Cost Recognized                                                                   | 6              | 8              | 21             | 23             |           |           |            |   |   |   |   |   |   |
| Contribution to the Supplemental Trust shares                                                                |           |           |           |           |           |           |            |   |   |   |   |   | 25.5 |
| Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year                        |           |           | $ 66           |           |           |           |            |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Contribution To The Trust Shares
+ References
No definition available.
+ Details**Name:**nav_ContributionToTrustShares**Namespace Prefix:**nav_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The increase in the fair value of plan assets from contributions made by the employer.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
If applicable, the cost of providing special or contractual termination benefits recognized during the period on the projected benefit obligation.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (q) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920
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| X                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of employer's best estimate of contributions expected to be paid to the plan in the current remaining fiscal period.
+ References
No definition available.
+ Details**Name:**us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInCurrentFiscalYear**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the cost recognized during the period for defined contribution plans.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942
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