Computation of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 02, 2015	Sep. 26, 2014	Oct. 02, 2015	Sep. 26, 2014
Numerators:
Income from continuing operations	$ 6,568	$ 5,119	$ 29,525	$ 11,381
Loss from discontinued operations		(594)	(13)	(5,138)
Less: Income attributable to noncontrolling interest				(10)
Loss from discontinued operations attributable to GSI Group Inc.		(594)	(13)	(5,148)
Net income attributable to GSI Group Inc.	$ 6,568	$ 4,525	$ 29,512	$ 6,233
Denominators:
Weighted average common shares outstanding—basic	34,599	34,389	34,578	34,333
Dilutive potential common shares	456	404	449	392
Weighted average common shares outstanding— diluted	35,055	34,793	35,027	34,725
Antidilutive common shares excluded from above				29
Basic Earnings (Loss) per Common Share:
From continuing operations	$ 0.19	$ 0.15	$ 0.85	$ 0.33
From discontinued operations		(0.02)	0.00	(0.15)
Basic earnings per share attributable to GSI Group Inc.	0.19	0.13	0.85	0.18
Diluted Earnings (Loss) per Common Share:
From continuing operations	0.19	0.15	0.84	0.33
From discontinued operations		(0.02)	0.00	(0.15)
Diluted earnings per share attributable to GSI Group Inc.	$ 0.19	$ 0.13	$ 0.84	$ 0.18