Borrowings - Schedule of Warehouse Credit Facilities (Details) - USD ($)	3 Months Ended
Mar. 31, 2026	Dec. 31, 2025
Line of Credit Facility [Line Items]
Outstanding borrowings	$ 1,951,762,000	$ 1,860,774,000
Revolving Credit Facility \| Upstart Auto Warehouse Trust 2 \| Variable Interest Entity, Primary Beneficiary
Line of Credit Facility [Line Items]
Total borrowing capacity	150,000,000
Collateral	0	3,000
Outstanding borrowings	$ 0	1,000
Revolving Credit Facility \| Upstart Auto Warehouse Trust 2 \| Variable Interest Entity, Primary Beneficiary \| Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Revolving Credit Facility \| Upstart Auto Warehouse Trust 2 \| Variable Interest Entity, Primary Beneficiary \| Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate	4.00%
Revolving Credit Facility \| Upstart Loan Trust
Line of Credit Facility [Line Items]
Total borrowing capacity	$ 325,000,000.0
Collateral	194,751,000	56,255,000
Outstanding borrowings	$ 109,000,000	23,025,000
Revolving Credit Facility \| Upstart Loan Trust \| Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.80%
Revolving Credit Facility \| Upstart Loan Trust \| Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate	3.40%
Revolving Credit Facility \| Upstart Small Dollar Loan Trust
Line of Credit Facility [Line Items]
Basis spread on variable rate	5.50%
Total borrowing capacity	$ 100,000,000.0
Collateral	135,404,000	128,973,000
Outstanding borrowings	$ 74,307,000	74,307,000
Commitment fee threshold	75.00%
Revolving Credit Facility \| Warehouse credit facilities
Line of Credit Facility [Line Items]
Total borrowing capacity	$ 575,000,000
Collateral	330,155,000	185,231,000
Outstanding borrowings	$ 183,307,000	$ 97,333,000
Revolving Credit Facility \| Warehouse credit facilities \| Maximum
Line of Credit Facility [Line Items]
Unused commitment fee	1.00%
Revolving Credit Facility \| ULT Warehouse Credit Facility - UnCommitted
Line of Credit Facility [Line Items]
Total borrowing capacity	$ 150,000,000.0

Line of Credit Facility, Unused Capacity, Commitment Fee Threshold, Percentage Of Borrowing Base

No definition available.

Name:	upst_LineOfCreditFacilityUnusedCapacityCommitmentFeeThresholdPercentageOfBorrowingBase
Namespace Prefix:	upst_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Long-Term And Short-Term Debt, Gross

+ References

No definition available.

+ Details

Name:	upst_LongTermAndShortTermDebtGross
Namespace Prefix:	upst_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of assets pledged to secure a debt instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477734/942-470-50-3

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480738/235-10-S50-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(f))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481544/470-10-50-6

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 470
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481544/470-10-50-6

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=upst_UAWTWarehouseCreditFacilityIIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=upst_ULTWarehouseCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=upst_UpstartSmallDollarLoanTrustMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=upst_WarehouseCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=upst_ULTWarehouseCreditFacilityUnCommittedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: