Computation of Basic and Diluted Earnings (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Oct. 02, 2015	Jul. 03, 2015	Apr. 03, 2015	Dec. 31, 2014	Sep. 26, 2014	Jun. 27, 2014	Mar. 28, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerators:
Income (loss) from continuing operations										$ 35,628	$ (16,909)	$ 9,977
Consolidated loss from discontinued operations										(13)	(7,333)	(2,646)
Less: income attributable to noncontrolling interest								$ (3)	$ (7)		(10)	(22)
Loss from discontinued operations attributable to GSI Group Inc.										(13)	(7,343)	(2,668)
Net income (loss) attributable to GSI Group Inc.						$ (30,485)	$ 4,525	$ 725	$ 983	$ 35,615	$ (24,252)	$ 7,309
Denominators:
Weighted average common shares outstanding—basic										34,579	34,352	34,073
Dilutive potential common shares	[1]									248		323
Weighted average common shares outstanding— diluted										34,827	34,352	34,396
Antidilutive common shares excluded from above											439	246
Basic Earnings (Loss) per Common Share:
From continuing operations		$ 0.18	$ 0.19	$ 0.56	$ 0.10	$ (0.82)	$ 0.15	$ 0.10	$ 0.08	$ 1.03	$ (0.49)	$ 0.29
From discontinued operations				0.00		(0.06)	(0.02)	(0.08)	(0.05)	0.00	(0.21)	(0.08)
Basic earnings (loss) per share attributable to GSI Group Inc.		0.18	0.19	0.56	0.10	(0.88)	0.13	0.02	0.03	1.03	(0.70)	0.21
Diluted Earnings (Loss) per Common Share:
From continuing operations		0.18	0.19	0.56	0.10	(0.82)	0.15	0.10	0.08	1.02	(0.49)	0.29
From discontinued operations				0.00		(0.06)	(0.02)	(0.08)	(0.05)	0.00	(0.21)	(0.08)
Diluted earnings (loss) per share attributable to GSI Group Inc.		$ 0.18	$ 0.19	$ 0.56	$ 0.10	$ (0.88)	$ 0.13	$ 0.02	$ 0.03	$ 1.02	$ (0.70)	$ 0.21

[1] Due to the Company’s net loss position for the year ended December 31, 2014, all potentially dilutive shares were excluded as their effect would have been anti-dilutive.