Net Income (Loss) Per Share Attributable to Upstart Holdings, Inc. Common Stockholders - Calculation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Numerator:
Net income (loss) attributable to Upstart Holdings, Inc. common stockholders	$ (108,665)	$ 135,443	$ 5,983
Less: noncumulative dividends to preferred stockholders	0	0	(5,983)
Net income (loss) attributable to common stockholders, basic	(108,665)	135,443	0
Net income (loss) attributable to common stockholders, diluted	$ (108,665)	$ 135,443	$ 0
Denominator:
Weighted-average common shares outstanding used to calculate net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, basic (in shares)	82,771,268	78,106,359	17,513,670
Weighted-average effect of dilutive securities (in shares)	0	16,666,282	0
Weighted-average common shares outstanding used to calculate net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, diluted (in shares)	82,771,268	94,772,641	17,513,670
Net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, basic (in dollars per share)	$ (1.31)	$ 1.73	$ 0
Net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, diluted (in dollars per share)	$ (1.31)	$ 1.43	$ 0