Net Income (Loss) Per Share Attributable to Upstart Holdings, Inc. Common Stockholders - Antidilutive Securities (Details) - shares	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	24,596,844	4,804,812
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	13,881,292	718,911
Unvested RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	8,222,931	1,077,703
Unvested PRSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	0	687,500
Purchase rights committed under the ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	174,543	2,620
Convertible debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total (in shares)	2,318,078	2,318,078