Net Income Per Share - Basic and Diluted Net Income (loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2019	Jun. 29, 2018	Jun. 28, 2019	Jun. 29, 2018
Numerator:
Net income (loss) attributable to Ultra Clean Holdings, Inc.	$ (202)	$ 18,960	$ 403	$ 43,701
Shares used in computation — basic:
Weighted average common shares outstanding	39,399	38,802	39,261	37,763
Shares used in computation — diluted:
Shares used in computing basic net income per share	39,399	38,802	39,261	37,763
Dilutive effect of common shares outstanding subject to repurchase		489	294	649
Dilutive effect of options outstanding		6	1	6
Weighted average shares used in computing diluted net income (loss) per share	39,399	39,297	39,556	38,418
Net income (loss) per share attributable to Ultra Clean Holdings, Inc. — basic	$ (0.01)	$ 0.49	$ 0.01	$ 1.16
Net income (loss) per share attributable to Ultra Clean Holdings, Inc. — diluted	$ (0.01)	$ 0.48	$ 0.01	$ 1.14