Net Income (Loss) Per Share Attributable to Upstart Holdings, Inc. Common Stockholders - Antidilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	21,700,781	3,285,537	19,675,223
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	12,547,010	461,157	19,600,223
Unvested RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	6,046,796	506,302	0
Unvested PRSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	687,500	0	0
Purchase rights committed under the ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	101,397	0	0
Convertible debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	2,318,078	2,318,078	75,000