v3.26.3
Summary of Significant Accounting Policies - Narrative (Details)
$ / shares in Units, $ in Millions
 9 Months Ended
May 08, 2026
shares
May 05, 2026
Sep. 30, 2026
USD ($)
segment
$ / shares
shares
Aug. 14, 2026
USD ($)
Jul. 27, 2026
USD ($)
Jul. 26, 2026
USD ($)
May 07, 2026
shares
May 06, 2026
shares
Dec. 31, 2025
$ / shares
shares
Segment Reporting [Line Items]                  
Stockholders' equity note, stock split, conversion ratio 0.2 0.2              
Stock split, number of additional shares received for each share held (in shares) | shares               4  
Number of operating segments | segment     1            
Number of reportable segments | segment     1            
MPSA | Ally Parties | Consumer Loan                  
Segment Reporting [Line Items]                  
Transfer of financial assets accounted for as sales, new total eligible amount to be sold | $     $ 8,000   $ 8,000 $ 6,000      
Definitive Loan Purchase Agreements | Ally Parties | Consumer Loan                  
Segment Reporting [Line Items]                  
Transfer of financial assets accounted for as sales, new total eligible amount to be sold | $     $ 500            
Line of Credit | Secured Debt | Term Loan B                  
Segment Reporting [Line Items]                  
Total principal amount | $       $ 1,660          
Class A Common Stock                  
Segment Reporting [Line Items]                  
Common stock, shares authorized (in shares) | shares 2,500,000,000   2,500,000,000       500,000,000   2,500,000,000
Common stock, par value (in dollars per share) | $ / shares     $ 0.001           $ 0.001
Class B Common Stock                  
Segment Reporting [Line Items]                  
Common stock, shares authorized (in shares) | shares 625,000,000   625,000,000       125,000,000   625,000,000
Common stock, par value (in dollars per share) | $ / shares     $ 0.001           $ 0.001