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		9 Months Ended
	Sep. 28, 2018	Sep. 29, 2017	Sep. 28, 2018	Sep. 29, 2017
Numerator:
Net income (loss) attributable to Ultra Clean Holdings, Inc.	$ (5,997)	$ 19,716	$ 37,704	$ 54,236
Shares used in computation — basic:
Weighted average common shares outstanding	38,930	33,540	38,152	33,342
Shares used in computation — diluted:
Shares used in computing basic net income per share	38,930	33,540	38,152	33,342
Dilutive effect of common shares outstanding subject to repurchase		813	588	856
Dilutive effect of options outstanding		7	5	18
Weighted average shares used in computing diluted net income per share	38,930	34,360	38,745	34,216
Net income per share attributable to Ultra Clean Holdings, Inc. — basic	$ (0.15)	$ 0.59	$ 0.99	$ 1.63
Net income per share attributable to Ultra Clean Holdings, Inc. — diluted	$ (0.15)	$ 0.57	$ 0.97	$ 1.59