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v3.19.2

Segment Reporting - Core Earnings Adjustments to GAAP (Parenthetical) (Detail) | 6 Months Ended
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Jun. 30, 2019 USD ($)
Segment Reporting [Abstract] |
Amount that will be equal to mark-to-market gain or loss over the life of the contract | $ 0

X
- Definition Mark-to-market gain or loss over life of contract. + References No definition available. + Details
Name: | navi_MarkToMarketGainOrLossOverLifeOfContract
Namespace Prefix: | navi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_SegmentReportingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration