Fair Value Measurement - Loans Rollforward (Details) - Fair Value, Inputs, Level 3 - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Loans (at fair value)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 623,763	$ 82,311	$ 252,477	$ 78,460
Reclassification of loans			0	0
Purchases of loans	375,181	92,184	1,514,822	173,043
Sale of loans	(232,302)	(34,429)	(866,984)	(101,330)
Purchase of loans for immediate resale	722,080	2,458,757	5,519,116	5,872,988
Immediate resale	(722,080)	(2,458,757)	(5,519,116)	(5,872,988)
Repayments received	(46,447)	(7,956)	(131,760)	(19,120)
Changes in fair value recorded in earnings	(21,059)	(2,889)	(73,376)	(1,921)
Other changes	1,319	404	5,276	493
Fair value, ending balance	700,455	129,625	700,455	129,625
Loan Held For Sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	605,319	26,741	142,685	60,232
Reclassification of loans			103,679	(26)
Purchases of loans	333,779	41,994	1,459,544	80,305
Sale of loans	(232,302)	(34,429)	(866,984)	(101,330)
Purchase of loans for immediate resale	722,080	2,458,757	5,519,116	5,872,988
Immediate resale	(722,080)	(2,458,757)	(5,519,116)	(5,872,988)
Repayments received	(43,748)	(1,831)	(128,023)	(6,867)
Changes in fair value recorded in earnings	(16,499)	(1,313)	(68,329)	(961)
Other changes	629	46	4,606	(145)
Fair value, ending balance	647,178	31,208	647,178	31,208
Loans Held For Investment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	18,444	55,570	109,792	18,228
Reclassification of loans			(103,679)	26
Purchases of loans	41,402	50,190	55,278	92,738
Sale of loans	0	0	0	0
Purchase of loans for immediate resale	0	0	0	0
Immediate resale	0	0	0	0
Repayments received	(2,699)	(6,125)	(3,737)	(12,253)
Changes in fair value recorded in earnings	(4,560)	(1,576)	(5,047)	(960)
Other changes	690	358	670	638
Fair value, ending balance	$ 53,277	$ 98,417	$ 53,277	$ 98,417

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Immediate Resale to Investors

No definition available.

Name:	upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetImmediateResaleToInvestors
Namespace Prefix:	upst_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Other Increase (Decrease)

+ References

No definition available.

+ Details

Name:	upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOtherIncreaseDecrease
Namespace Prefix:	upst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchase Of Loans For Immediate Resale To Investors

+ References

No definition available.

+ Details

Name:	upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchaseOfLoansForImmediateResaleToInvestors
Namespace Prefix:	upst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Reclassification

+ References

No definition available.

+ Details

Name:	upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetReclassification
Namespace Prefix:	upst_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Repayments Received

+ References

No definition available.

+ Details

Name:	upst_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetRepaymentsReceived
Namespace Prefix:	upst_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_LoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=upst_LoanHeldForSaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=upst_LoansHeldForInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: