Segment Reporting - Core Earnings Adjustments to GAAP (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Adjustments Required To Reconcile Core Earnings [Line Items]
Net Income (Loss)	$ 25	$ 14	$ 42	$ 11
Net tax effect	(14)	(4)	(29)	(1)
Operating Segments [Member]
Adjustments Required To Reconcile Core Earnings [Line Items]
Net Income (Loss)	27	21	47	47
Net tax effect	(9)	(6)	(15)	(14)
Total Adjustments [Member] \| Adjustments [Member]
Adjustments Required To Reconcile Core Earnings [Line Items]
Net Income (Loss)	2	7	5	36
Core Earnings adjustments to GAAP	(3)	9	(9)	49
Net tax effect	5	(2)	14	(13)
Net Impact of Derivative Accounting [Member] \| Total Adjustments [Member] \| Adjustments [Member]
Adjustments Required To Reconcile Core Earnings [Line Items]
Core Earnings adjustments to GAAP	(3)	8	(13)	47
Location, Statement of Income, Balance [Axis]: navi:GoodwillAndAcquiredIntangibleAssetsImpairmentAndAmortizationExpense \| Total Adjustments [Member] \| Adjustments [Member]
Adjustments Required To Reconcile Core Earnings [Line Items]
Core Earnings adjustments to GAAP	$ 0	$ 1	$ 4	$ 2

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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+ Details

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

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+ Details