Loss Per Share - Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Numerator
Net loss	$ (14,092)	$ (13,563)	$ (18,095)	$ (27,655)	$ (32,966)
Less: Net loss attributable to noncontrolling interests	(10,691)		(14,440)	(21,062)	(26,206)
Net loss attributable to Viant Technology Inc.	$ (3,401)		$ (3,655)	$ (6,593)	$ (6,760)
Weighted-average shares of Class A common stock outstanding:
Weighted-average shares of Class A common stock outstanding — basic	14,114		11,500	13,962	11,500
Weighted-average shares of Class A common stock outstanding — diluted	14,114		11,500	13,962	11,500
Basic	$ (0.24)		$ (0.32)	$ (0.47)	$ (0.59)
Diluted	$ (0.24)		$ (0.32)	$ (0.47)	$ (0.59)
Anti-dilutive shares excluded from loss per share of Class A common stock—diluted:
Total shares excluded from loss per share of Class A common stock—diluted	55,761		53,687	55,761	53,687
Class A Common Stock
Weighted-average shares of Class A common stock outstanding:
Weighted-average shares of Class A common stock outstanding — basic	14,114		11,500	13,962	11,500
Weighted-average shares of Class A common stock outstanding — diluted	14,114		11,500	13,962	11,500
Class B Common Stock
Anti-dilutive shares excluded from loss per share of Class A common stock—diluted:
Total shares excluded from loss per share of Class A common stock—diluted	47,082		47,436	47,082	47,436
Restricted Stock Units
Anti-dilutive shares excluded from loss per share of Class A common stock—diluted:
Total shares excluded from loss per share of Class A common stock—diluted	4,781		6,184	4,781	6,184
Nonqualified Stock Options
Anti-dilutive shares excluded from loss per share of Class A common stock—diluted:
Total shares excluded from loss per share of Class A common stock—diluted	3,898		67	3,898	67