v3.26.3
Borrowings - Narrative (Details)
1 Months Ended 3 Months Ended 9 Months Ended
Aug. 15, 2026
USD ($)
Aug. 31, 2025
USD ($)
Aug. 31, 2021
USD ($)
Sep. 30, 2026
USD ($)
Sep. 30, 2025
USD ($)
Sep. 30, 2024
USD ($)
Sep. 30, 2026
USD ($)
day
Sep. 30, 2025
USD ($)
Nov. 03, 2026
USD ($)
Jun. 05, 2026
USD ($)
Jun. 04, 2026
USD ($)
Apr. 22, 2026
USD ($)
Apr. 21, 2026
USD ($)
Dec. 31, 2025
USD ($)
Jun. 30, 2025
USD ($)
Nov. 30, 2024
USD ($)
Schedule of Borrowings [Line Items]                                
Total payments due       $ 2,054,164,000     $ 2,054,164,000             $ 1,860,774,000    
Proceeds from convertible notes issuance, net of debt issuance costs paid to lender             0 $ 678,270,000                
Gain on debt extinguishment       0 $ 7,246,000   0 7,246,000                
Convertible senior notes                                
Schedule of Borrowings [Line Items]                                
Total payments due       1,621,250,000     $ 1,621,250,000             1,687,794,000    
Conversion price maximum threshold             130.00%                  
Trading days | day             20                  
Consecutive trading days | day             30                  
Redemption price (in percent)     100.00%       100.00%                  
Interest expense       0 0   $ 10,300,000 $ 10,700,000                
Convertible senior notes | Conversion Period One                                
Schedule of Borrowings [Line Items]                                
Conversion price maximum threshold             130.00%                  
Trading days | day             20                  
Consecutive trading days | day             30                  
Convertible senior notes | Conversion Period Two                                
Schedule of Borrowings [Line Items]                                
Trading days | day             5                  
Consecutive trading days | day             5                  
Percentage of the last reported sale price             98.00%                  
Risk retention financing facility | Secured debt                                
Schedule of Borrowings [Line Items]                                
Total payments due       105,310,000     $ 105,310,000             75,647,000    
Aggregate principal amount                       $ 200,000,000.0 $ 100,000,000.0   $ 100,000,000.0  
Securities loaned, fair value of collateral       $ 104,000,000.0     $ 104,000,000.0                  
Weighted average interest rate       0.40%     0.40%                  
Risk retention financing facility | Secured debt | Location, Statement of Financial Position, Balance [Axis]: us-gaap:NotesReceivableFairValueDisclosure                                
Schedule of Borrowings [Line Items]                                
Securities loaned, fair value of collateral       $ 102,600,000     $ 102,600,000                  
2026 Notes | Convertible senior notes                                
Schedule of Borrowings [Line Items]                                
Aggregate principal amount     $ 661,300,000                          
Stated interest rate     0.25%                          
Repurchase amount   $ 232,600,000       $ 334,200,000                    
Additional repurchased amount           27,900,000                    
Gain on debt extinguishment $ 0       $ 7,200,000 33,400,000                    
Principal repaid $ 66,500,000                              
Gross debt issuance cost     $ 15,700,000                          
Effective interest rate     0.70%                          
2029 Notes | Convertible senior notes                                
Schedule of Borrowings [Line Items]                                
Aggregate principal amount           $ 431,300,000                    
Stated interest rate           2.00%                    
Proceeds from convertible notes issuance, net of debt issuance costs paid to lender           $ 302,400,000                    
Gross debt issuance cost           $ 10,400,000                    
Effective interest rate           2.50%                    
2030 Notes | Convertible senior notes                                
Schedule of Borrowings [Line Items]                                
Aggregate principal amount                               $ 500,000,000.0
Stated interest rate                               1.00%
Gross debt issuance cost                               $ 11,800,000
Effective interest rate                               1.40%
2032 Notes | Convertible senior notes                                
Schedule of Borrowings [Line Items]                                
Aggregate principal amount   $ 690,000,000.0                            
Stated interest rate   0.00%                            
Proceeds from convertible notes issuance, net of debt issuance costs paid to lender   $ 224,200,000                            
Gross debt issuance cost   $ 14,400,000                            
Effective interest rate   0.30%                            
Revolving Credit Facility | Upstart Loan Trust                                
Schedule of Borrowings [Line Items]                                
Maximum borrowing capacity       400,000,000.0     400,000,000.0     $ 400,000,000.0 $ 325,000,000.0          
Total payments due       $ 253,297,000     $ 253,297,000     $ 175,000,000.0       $ 23,025,000    
Revolving Credit Facility | UAWT Warehouse Credit Facility II - Uncommitted | Variable Interest Entity, Primary Beneficiary | Subsequent Event                                
Schedule of Borrowings [Line Items]                                
Maximum borrowing capacity                 $ 150,000,000