Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 03, 2015	Jun. 27, 2014	Jul. 03, 2015	Jun. 27, 2014
Numerators:
Income from continuing operations	$ 19,511	$ 3,406	$ 22,957	$ 6,262
Operating loss from discontinued operations, net of tax	(13)	(2,678)	(13)	(4,544)
Less: Income attributable to noncontrolling interest		(3)		(10)
Loss from discontinued operations attributable to GSI Group Inc.	(13)	(2,681)	(13)	(4,554)
Net income attributable to GSI Group Inc.	$ 19,498	$ 725	$ 22,944	$ 1,708
Denominators:
Weighted average common shares outstanding—basic	34,630	34,378	34,567	34,304
Dilutive potential common shares	399	329	447	386
Weighted average common shares outstanding— diluted	35,029	34,707	35,014	34,690
Antidilutive common shares excluded from above				44
Basic Earnings (Loss) per Common Share:
From continuing operations	$ 0.56	$ 0.10	$ 0.66	$ 0.18
From discontinued operations	0.00	(0.08)	0.00	(0.13)
Basic earnings per share attributable to GSI Group Inc.	0.56	0.02	0.66	0.05
Diluted Earnings (Loss) per Common Share:
From continuing operations	0.56	0.10	0.66	0.18
From discontinued operations	0.00	(0.08)	0.00	(0.13)
Diluted earnings per share attributable to GSI Group Inc.	$ 0.56	$ 0.02	$ 0.66	$ 0.05