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Revenue - Loan Servicing Rights (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Revenue from Contract with Customer [Abstract] |  |
Net gain (loss) related to loan servicing rights | $ (67) | $ 1,459

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- Definition Net Gain (Loss) Related To Loan Servicing Rights + References No definition available. + Details
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Namespace Prefix: | upst_
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Balance Type: | credit
Period Type: | duration

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