Stockholders’ Equity - Schedule of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 39,955	$ 34,155	$ 119,232	$ 99,497
Income Statement Location [Axis]: us-gaap:GeneralAndAdministrativeExpense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	17,325	11,864	46,279	34,976
Income Statement Location [Axis]: us-gaap:ResearchAndDevelopmentExpense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	18,180	17,243	58,140	49,493
Income Statement Location [Axis]: us-gaap:SellingAndMarketingExpense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	2,630	3,371	9,034	9,680
Income Statement Location [Axis]: upst:CustomerOperationsExpense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 1,820	$ 1,677	$ 5,779	$ 5,348