v3.26.3
Segment Reporting - Segment Results and Reconciliations to GAAP - Core Earnings Adjustments (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]        
Net interest income (loss) after provisions for loan losses $ (112) $ (26) $ 87 $ 164
Goodwill and acquired intangible asset impairment and amortization 0 1 5 2
Income tax expense (benefit) (2) (31) 26 (29)
Net income (loss) 15 (86) 56 (75)
Total Adjustments [Member] | Adjustments [Member]        
Segment Reporting [Line Items]        
Net interest income (loss) after provisions for loan losses (4) 4 (11) 21
Total other income (5) 4 (11) 34
Goodwill and acquired intangible asset impairment and amortization 0 (1) (5) (2)
Total Core Earnings adjustments to GAAP (9) 9 (17) 57
Income tax expense (benefit) 3 6 (11) 18
Net income (loss) (12) 3 (6) 39
Total Adjustments [Member] | Adjustments [Member] | Location, Statement of Income, Balance [Axis]: navi:GoodwillAndAcquiredIntangibleAssetsImpairmentAndAmortizationExpense        
Segment Reporting [Line Items]        
Net interest income (loss) after provisions for loan losses 0 0 0 0
Total other income 0 0 0 0
Goodwill and acquired intangible asset impairment and amortization 0 (1) (5) (2)
Total Core Earnings adjustments to GAAP 0 1 5 2
Net Impact of Derivative Accounting [Member] | Total Adjustments [Member] | Adjustments [Member]        
Segment Reporting [Line Items]        
Net interest income (loss) after provisions for loan losses (4) 4 (11) 21
Total other income (5) 4 (11) 34
Goodwill and acquired intangible asset impairment and amortization 0 0 0 0
Total Core Earnings adjustments to GAAP $ (9) $ 8 $ (22) $ 55