Variable Interest Entities - Schedule of Financial Assets and Liabilities from Variable Interest Entities (Details) - USD ($) $ in Thousands		Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Variable Interest Entity [Line Items]
Assets	[1]	$ 1,808,980		$ 2,017,100
Liabilities	[1]	1,213,444		1,381,795
Net Assets		595,536	$ 594,710	635,305	$ 640,764	$ 638,145	$ 672,435
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Assets		689,614		1,195,393
Liabilities		275,348		530,952
Net Assets		414,266		664,441
Variable Interest Entity, Primary Beneficiary | Consolidated securitization
Variable Interest Entity [Line Items]
Assets		125,255		187,258
Liabilities		100,337		141,420
Net Assets		24,918		45,838
Variable Interest Entity, Primary Beneficiary | Consolidated warehouse entities
Variable Interest Entity [Line Items]
Assets		353,922		645,455
Liabilities		174,067		388,681
Net Assets		179,855		256,774
Variable Interest Entity, Primary Beneficiary | Other consolidated VIEs
Variable Interest Entity [Line Items]
Assets		210,437		362,680
Liabilities		944		851
Net Assets		209,493		361,829
Variable Interest Entity, Not Primary Beneficiary | Securitizations
Variable Interest Entity [Line Items]
Assets		293,412		445,929
Liabilities		210,965		319,357
Net Assets		82,447		126,572
Maximum Exposure to Losses		$ 13,972		$ 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 September 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,September 30,20232024AssetsCash$1,603 $2,041 Restricted cash23,450 50,231 Loans (at fair value)1,147,423 624,318  Other assets (includes $5,958 and $2,139 at fair value as of December 31, 2023 and September 30, 2024, respectively) 22,917 13,024 Total assets$1,195,393 $689,614 LiabilitiesPayable to investors$121 $154 Borrowings387,440 170,085 Payable to securitization note holders (at fair value)141,416 100,335 Accrued expenses and other liabilities1,975 4,774 Total liabilities530,952 275,348 Total net assets$664,441 $414,266