Variable Interest Entities - Schedule of Financial Assets and Liabilities from Variable Interest Entities (Details) - USD ($) $ in Thousands		Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Variable Interest Entity [Line Items]
Assets	[1]	$ 1,820,224		$ 2,017,100
Liabilities	[1]	1,225,514		1,381,795
Net Assets		594,710	$ 612,798	635,305	$ 638,145	$ 627,208	$ 672,435
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets		882,855		1,195,393
Liabilities		374,905		530,952
Net Assets		507,950		664,441
Variable Interest Entity, Primary Beneficiary | Consolidated securitization
Variable Interest Entity [Line Items]
Assets		142,540		187,258
Liabilities		113,656		141,420
Net Assets		28,884		45,838
Variable Interest Entity, Primary Beneficiary | Consolidated warehouse entities
Variable Interest Entity [Line Items]
Assets		513,755		645,455
Liabilities		260,065		388,681
Net Assets		253,690		256,774
Variable Interest Entity, Primary Beneficiary | Other consolidated VIEs
Variable Interest Entity [Line Items]
Assets		226,560		362,680
Liabilities		1,184		851
Net Assets		225,376		361,829
Variable Interest Entity, Not Primary Beneficiary | Securitizations
Variable Interest Entity [Line Items]
Assets		340,186		445,929
Liabilities		246,303		319,357
Net Assets		93,883		126,572
Maximum Exposure to Losses		$ 15,296		$ 20,885

[1] The following table presents information on assets and liabilities related to variable interest entities (“VIEs”) that are consolidated by Upstart Holdings, Inc. at December 31, 2023 and June 30, 2024. The assets in the table below may only be used to settle obligations of consolidated VIEs and are in excess of those obligations. The liabilities in the table below include liabilities for which creditors do not have recourse to the general credit of Upstart Holdings, Inc. The assets and liabilities in the table below include third-party assets and liabilities of consolidated VIEs and exclude intercompany balances that eliminate in consolidation. December 31,June 30,20232024AssetsCash$1,603 $524 Restricted cash23,450 61,084 Loans (at fair value)1,147,423 803,111  Other assets (includes $5,958 and $5,123 at fair value as of December 31, 2023 and June 30, 2024, respectively) 22,917 18,136 Total assets$1,195,393 $882,855 LiabilitiesPayable to investors$121 $14 Borrowings387,440 258,203 Payable to securitization note holders (at fair value)141,416 113,652 Accrued expenses and other liabilities1,975 3,036 Total liabilities530,952 374,905 Total net assets$664,441 $507,950