Computation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 28, 2012	Sep. 30, 2011	Sep. 28, 2012	Sep. 30, 2011
Numerators:
Income from continuing operations	$ 2,055	$ 7,327	$ 7,599	$ 19,072
Income (loss) from discontinued operations, net of tax	(4,570)	1,489	(3,834)	6,081
Less: Net (income) loss attributable to noncontrolling interest	(19)	29	(45)	(35)
Net income (loss) attributable to GSI Group Inc.	$ (2,534)	$ 8,845	$ 3,720	$ 25,118
Denominators:
Weighted average common shares outstanding-basic	33,803	33,489	33,755	33,472
Dilutive potential common shares	109	99	159	100
Weighted average common shares outstanding-diluted	33,912	33,588	33,914	33,572
Antidilutive common shares excluded from above	424	183	141	118
Basic Earnings (Loss) per Common Share:
From continuing operations	$ 0.06	$ 0.22	$ 0.23	$ 0.57
From discontinued operations	$ (0.14)	$ 0.04	$ (0.11)	$ 0.18
Basic earnings (loss) per share attributable to GSI Group Inc.	$ (0.07)	$ 0.26	$ 0.11	$ 0.75
Diluted Earnings (Loss) per Common Share:
From continuing operations	$ 0.06	$ 0.22	$ 0.22	$ 0.57
From discontinued operations	$ (0.14)	$ 0.04	$ (0.11)	$ 0.18
Diluted earnings (loss) per share attributable to GSI Group Inc.	$ (0.07)	$ 0.26	$ 0.11	$ 0.75