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Borrowings - Narrative (Details) |  |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended |
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Jun. 27, 2025 | Jun. 26, 2025 | Sep. 16, 2024 | Aug. 20, 2021 | Nov. 30, 2024 USD ($) $ / shares | Sep. 30, 2024 USD ($) day $ / shares | Aug. 31, 2021 USD ($) day $ / shares | Jun. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) $ / shares | Jun. 30, 2025 USD ($) | Dec. 31, 2024 USD ($)
Schedule of Borrowings [Line Items] |  |  |  |  |  |  |  |  |  |  |
Outstanding borrowings |  |  |  |  |  |  |  | $ 1,449,649,000 |  | $ 1,449,649,000 | $ 1,425,984,000
Securities loaned, fair value of collateral |  |  |  |  |  |  |  | 29,900,000 |  | 29,900,000 | 0
Convertible senior notes |  |  |  |  |  |  |  |  |  |  |
Schedule of Borrowings [Line Items] |  |  |  |  |  |  |  |  |  |  |
Outstanding borrowings |  |  |  |  |  |  |  | 1,230,379,000 |  | 1,230,379,000 | 1,230,379,000
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 |  | 7,200,000 |
Accrued interest |  |  |  |  |  |  |  | 0 |  | 0 | 3,300,000
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] |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | 100,000,000.0 |  | 100,000,000.0 |
Outstanding borrowings |  |  |  |  |  |  |  | 29,731,000 |  | 29,731,000 | 0
Securities loaned, fair value of collateral |  |  |  |  |  |  |  | $ 29,900,000 |  | $ 29,900,000 |
Weighted average interest rate |  |  |  |  |  |  |  | 0.40% |  | 0.40% |
2026 Notes | Convertible senior notes |  |  |  |  |  |  |  |  |  |  |
Schedule of Borrowings [Line Items] |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  | $ 661,300,000 |  |  |  |
Stated interest rate |  |  |  |  |  |  | 0.25% |  |  |  |
Repurchase amount |  |  |  |  |  | $ 334,200,000 |  |  | $ 334,200,000 |  |
Additional repurchased amount |  |  |  |  |  | 27,900,000 |  |  | 27,900,000 |  |
Gain on debt extinguishment |  |  |  |  |  |  |  |  | 33,400,000 |  |
Initial conversion rate (in shares) |  |  |  | 0.0035056 |  |  |  |  |  |  |
Initial conversion price (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 285.26 |  |  |  |
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 |  |  | $ 431,300,000 |  |
Stated interest rate |  |  |  |  |  | 2.00% |  |  | 2.00% |  |
Issuance of notes |  |  |  |  |  | $ 302,400,000 |  |  |  |  |
Initial conversion rate (in shares) |  |  | 0.0219029 |  |  |  |  |  |  |  |
Initial conversion price (in dollars per share) | $ / shares |  |  |  |  |  | $ 45.66 |  |  | $ 45.66 |  |
Gross debt issuance cost |  |  |  |  |  | $ 10,400,000 |  |  | $ 10,400,000 |  |
Effective interest rate |  |  |  |  |  | 2.50% |  |  | 2.50% |  |
2030 Notes | Convertible senior notes |  |  |  |  |  |  |  |  |  |  |
Schedule of Borrowings [Line Items] |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 500,000,000.0 |  |  |  |  |  |
Stated interest rate |  |  |  |  | 1.00% |  |  |  |  |  |
Initial conversion rate (in shares) |  |  |  |  | 0.0108702 |  |  |  |  |  |
Initial conversion price (in dollars per share) | $ / shares |  |  |  |  | $ 91.99 |  |  |  |  |  |
Gross debt issuance cost |  |  |  |  | $ 11,800,000 |  |  |  |  |  |
Effective interest rate |  |  |  |  | 1.40% |  |  |  |  |  |
Revolving Credit Facility | Upstart Loan Trust |  |  |  |  |  |  |  |  |  |  |
Schedule of Borrowings [Line Items] |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate | 2.40% | 2.80% |  |  |  |  |  |  |  |  |
Outstanding borrowings |  |  |  |  |  |  |  | $ 115,232,000 |  | $ 115,232,000 | $ 34,217,000

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