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Segment Reporting - Core Earnings Adjustments to GAAP (Parenthetical) (Detail) | 3 Months Ended
--- | ---
Mar. 31, 2017 USD ($)
Segment Reporting [Abstract] |
Amount that will be equal to cumulative net unrealized gain or loss over the life of the contract | $ 0

X
- Definition Cumulative net unrealized gain or loss over the life of the contract. + References No definition available. + Details
Name: | navi_CumulativeNetUnrealizedGainOrLossOverLifeOfContract
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