Net Income (Loss) Per Share Attributable to Upstart Holdings, Inc. Common Stockholders - Calculation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Numerator:
Net income (loss)	$ (129,254)	$ 32,692
Denominator:
Weighted-average common shares outstanding used to calculate net income (loss) per share, basic (in shares)	81,911,433	84,230,445
Weighted-average effect of dilutive securities (in shares)	0	11,227,331
Weighted-average common shares outstanding used to calculate net income (loss) per share, diluted (in shares)	81,911,433	95,457,776
Net income (loss) per share, basic (in dollars per share)	$ (1.58)	$ 0.39
Net income (loss) per share, diluted (in dollars per share)	$ (1.58)	$ 0.34