Segment Reporting - Core Earnings Adjustments to GAAP (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Adjustments Required To Reconcile Core Earnings [Line Items]
Net Income (Loss)	$ 15	$ (86)	$ 56	$ (75)
Net tax effect	2	31	(26)	29
Operating Segments [Member]
Adjustments Required To Reconcile Core Earnings [Line Items]
Net Income (Loss)	3	(83)	50	(36)
Net tax effect	(1)	25	(15)	11
Total Adjustments [Member] \| Adjustments [Member]
Adjustments Required To Reconcile Core Earnings [Line Items]
Net Income (Loss)	(12)	3	(6)	39
Core Earnings adjustments to GAAP	(9)	9	(17)	57
Net tax effect	(3)	(6)	11	(18)
Net Impact of Derivative Accounting [Member] \| Total Adjustments [Member] \| Adjustments [Member]
Adjustments Required To Reconcile Core Earnings [Line Items]
Core Earnings adjustments to GAAP	(9)	8	(22)	55
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	$ 5	$ 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