Net Income Per Share - Calculation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Numerator:
Net income	$ 32,692	$ 10,102
Denominator:
Weighted-average common shares outstanding used to calculate net income per share attributable to Upstart Holdings, Inc. common stockholders, basic (in shares)	84,230,445	73,629,122
Weighted-average effect of dilutive securities (in shares)	11,227,331	17,820,449
Weighted-average common shares outstanding used to calculate net income per share attributable to Upstart Holdings, Inc. common stockholders, diluted (in shares)	95,457,776	91,449,571
Net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, basic (in dollars per share)	$ 0.39	$ 0.14
Net income (loss) per share attributable to Upstart Holdings, Inc. common stockholders, diluted (in dollars per share)	$ 0.34	$ 0.11