Net Income (Loss) Per Share Attributable to Upstart Holdings, Inc. Common Stockholders - Antidilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities were excluded from the computation of diluted net income per share attributable to Upstart Holdings, Inc. common stockholders because including them would have been anti-dilutive	3,285,537	19,675,223	64,771,660
Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities were excluded from the computation of diluted net income per share attributable to Upstart Holdings, Inc. common stockholders because including them would have been anti-dilutive	0	0	47,349,577
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities were excluded from the computation of diluted net income per share attributable to Upstart Holdings, Inc. common stockholders because including them would have been anti-dilutive	461,157	19,600,223	16,502,206
Unvested RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities were excluded from the computation of diluted net income per share attributable to Upstart Holdings, Inc. common stockholders because including them would have been anti-dilutive	506,302	0	0
ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities were excluded from the computation of diluted net income per share attributable to Upstart Holdings, Inc. common stockholders because including them would have been anti-dilutive	0	0	0
Warrants to purchase convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities were excluded from the computation of diluted net income per share attributable to Upstart Holdings, Inc. common stockholders because including them would have been anti-dilutive	0	0	600,208
Convertible debt
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities were excluded from the computation of diluted net income per share attributable to Upstart Holdings, Inc. common stockholders because including them would have been anti-dilutive	2,318,078	0	319,669
Warrants to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities were excluded from the computation of diluted net income per share attributable to Upstart Holdings, Inc. common stockholders because including them would have been anti-dilutive	0	75,000	0