Borrowings - Narrative (Details)			1 Months Ended		3 Months Ended	12 Months Ended
	Jun. 27, 2025	Jun. 26, 2025	Aug. 31, 2025 USD ($)	Aug. 31, 2021 USD ($)	Sep. 30, 2024 USD ($)	Dec. 31, 2025 USD ($) day	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Aug. 07, 2025 USD ($)	Aug. 06, 2025 USD ($)	Jun. 30, 2025 USD ($)	Nov. 30, 2024 USD ($)
Schedule of Borrowings [Line Items]
Total payments due						$ 1,860,774,000	$ 1,425,984,000
Proceeds from convertible notes issuance, net of debt issuance costs paid to lender						678,270,000	913,440,000	$ 0
Gain on debt extinguishment						7,246,000	33,361,000	0
Convertible senior notes
Schedule of Borrowings [Line Items]
Total payments due						$ 1,687,794,000	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						$ 14,200,000	4,500,000	0
Amortization of debt issuance costs						0	3,200,000	$ 3,100,000
Accrued interest						$ 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
Total payments due						$ 75,647,000	0
Outstanding borrowings						$ 75,900,000
Weighted average interest rate						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			$ 232,600,000		$ 334,200,000
Additional repurchased amount					27,900,000
Gain on debt extinguishment						$ 7,200,000	33,400,000
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			690,000,000
Gross debt issuance cost			$ 14,400,000
Effective interest rate			0.30%
Revolving Credit Facility | Upstart Loan Trust
Schedule of Borrowings [Line Items]
Basis spread on variable rate	2.40%	2.80%
Maximum borrowing capacity						325,000,000
Total payments due						23,025,000	34,217,000
Revolving Credit Facility | Upstart Auto Warehouse Trust 2 | Variable Interest Entity, Primary Beneficiary
Schedule of Borrowings [Line Items]
Maximum borrowing capacity						150,000,000			$ 150,000,000.0	$ 50,000,000.0
Total payments due						$ 1,000	$ 11,353,000