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, 2021	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net income (loss) attributable to Upstart Holdings, Inc. common stockholders	$ 135,443	$ 5,983	$ (466)
Less: noncumulative dividends to preferred stockholders	0	(5,983)	0
Net income (loss) attributable to common stockholders, basic	135,443	0	(466)
Add: adjustments to undistributed earnings to participating securities	0	0
Net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, diluted	$ 135,443	$ 0	$ (466)
Denominator:
Weighted-average common shares outstanding used to calculate net income per share attributable to Upstart Holdings, Inc. common stockholders, basic (in shares)	78,106,359	17,513,670	14,335,611
Weighted-average effect of dilutive securities (in shares)	16,666,282	0	0
Weighted-average common shares outstanding used to calculate net income per share attributable to Upstart Holdings, Inc. common stockholders, diluted (in shares)	94,772,641	17,513,670	14,335,611
Net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, basic (in dollars per share)	$ 1.73	$ 0	$ (0.03)
Net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, diluted (in dollars per share)	$ 1.43	$ 0	$ (0.03)