Acquisitions - Consideration Transferred (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Apr. 08, 2021	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Fair value of Upstart common stock issued to Prodigy stockholders		$ 0	$ 80,256	$ 0
Prodigy Software, Inc.
Business Acquisition [Line Items]
Fair value of Upstart common stock issued to Prodigy stockholders	$ 70,121
Cash paid to common and preferred stockholders, warrant holders, and vested option holders	17,151
Fair value of assumed Prodigy options attributable to pre-combination service period	889
Transactions costs paid by Upstart on behalf of Prodigy	883
Total purchase consideration	$ 89,044
Shares of common stock transferred (in shares)	568,539
Value of common stock transferred (in dollars per share)	$ 123.33
Shares held in escrow (in shares)	87,339
Cash payments held in escrow	$ 1,900