Segment Reporting - Core Earnings Adjustments to GAAP (Parenthetical) (Detail)	6 Months Ended
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

Mark-to-market gain or loss over life of contract.

No definition available.

Name:	navi_MarkToMarketGainOrLossOverLifeOfContract
Namespace Prefix:	navi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration