Computation of Basic and Diluted Earnings (Loss) Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 28, 2012	Jun. 29, 2012	Mar. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jul. 01, 2011	Apr. 01, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Amounts attributable to GSI Group Inc:
Income (loss) from continuing operations									$ 22,434		$ 21,648		$ (19,973)
Income (loss) from discontinued operations.									(2,896)		7,325		19,286
Net income (loss)	$ 15,818	$ (2,534)	$ 4,873	$ 1,381	$ 3,855	$ 8,845	$ 10,066	$ 6,207	$ 19,538		$ 28,973		$ (687)
Denominators:
Weighted average common shares outstanding-basic									33,775		33,481		23,703
Dilutive potential common shares									161	[1]	108	[1]
Weighted average common shares outstanding-diluted									33,936		33,589		23,703
Antidilutive common shares excluded from above									244		132		183
Basic Earnings (Loss) per Common Share:
From continuing operations	$ 0.44	$ 0.06	$ 0.13	$ 0.03	$ 0.08	$ 0.22	$ 0.21	$ 0.14	$ 0.66		$ 0.65		$ (0.84)
From discontinued operations	$ 0.03	$ (0.13)	$ 0.01	$ 0.01	$ 0.04	$ 0.04	$ 0.09	$ 0.05	$ (0.08)		$ 0.22		$ 0.81
Basic earnings (loss) per share attributable to GSI Group Inc.	$ 0.47	$ (0.07)	$ 0.14	$ 0.04	$ 0.12	$ 0.26	$ 0.30	$ 0.19	$ 0.58		$ 0.87		$ (0.03)
Diluted Earnings (Loss) per Common Share:
From continuing operations	$ 0.44	$ 0.06	$ 0.13	$ 0.03	$ 0.08	$ 0.22	$ 0.21	$ 0.14	$ 0.66		$ 0.64		$ (0.84)
From discontinued operations	$ 0.03	$ (0.13)	$ 0.01	$ 0.01	$ 0.03	$ 0.04	$ 0.09	$ 0.05	$ (0.08)		$ 0.22		$ 0.81
Diluted earnings (loss) per share attributable to GSI Group Inc.	$ 0.47	$ (0.07)	$ 0.14	$ 0.04	$ 0.11	$ 0.26	$ 0.30	$ 0.19	$ 0.58		$ 0.86		$ (0.03)

[1] Due to the Company's loss from continuing operations and net loss attributable to GSI Group Inc. for the year ended December 31, 2010, all potentially dilutive shares were excluded as their effect would have been anti-dilutive. The dilutive impact of 14 thousand shares for the year ended December 31, 2010 would not have an impact on earnings per common share from discontinued operations.