v3.26.1
Segment Reporting - Segment Results and Reconciliations to GAAP - Core Earnings Adjustments (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting [Line Items]        
Net interest income (loss) after provisions for loan losses $ 96 $ 91 $ 199 $ 191
Goodwill and acquired intangible asset impairment and amortization 0 1 4 2
Income tax expense (benefit) 14 4 29 1
Net income 25 14 42 11
Total Adjustments [Member] | Adjustments [Member]        
Segment Reporting [Line Items]        
Net interest income (loss) after provisions for loan losses (2) 3 (7) 17
Total other income (1) 5 (6) 30
Goodwill and acquired intangible asset impairment and amortization 0 (1) (4) (2)
Total Core Earnings adjustments to GAAP (3) 9 (9) 49
Income tax expense (benefit) (5) 2 (14) 13
Net income 2 7 5 36
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) (4) (2)
Total Core Earnings adjustments to GAAP 0 1 4 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 (2) 3 (7) 17
Total other income (1) 5 (6) 30
Goodwill and acquired intangible asset impairment and amortization 0 0 0 0
Total Core Earnings adjustments to GAAP $ (3) $ 8 $ (13) $ 47