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Condensed Consolidated Statements of Operations and Comprehensive Income (Parenthetical) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Income Statement [Abstract] |  |
Related party interest expense |  | $ 608
Fair value adjustments from related parties | $ 4,335 |

X
- Definition Fair Value Adjustments, Related Party + References No definition available. + Details
Name: | upst_FairValueAdjustmentsRelatedParty
Namespace Prefix: | upst_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_IncomeStatementAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of interest expense incurred on a debt or other obligation to related party. + References No definition available. + Details
Name: | us-gaap_InterestExpenseRelatedParty
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration