Quarterly Financial Information (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
Quarterly Financial Information [Line Items]
Sales	$ 66,413	$ 69,520	$ 70,379	$ 65,186	$ 65,480	$ 78,704	$ 82,609	$ 77,503	$ 271,498	$ 304,296	$ 285,892
Cost of goods sold	39,138	40,667	39,712	37,505	37,707	44,119	45,609	43,761	157,022	171,196	155,278
Gross profit	27,275	28,853	30,667	27,681	27,773	34,585	37,000	33,742	114,476	133,100	130,614
Operating expenses	23,978	25,061	25,570	24,860	23,988	23,761	25,537	23,966	99,469	97,252	94,587
Income from operations	3,297	3,792	5,097	2,821	3,785	10,824	11,463	9,776	15,007	35,848	36,027
Interest income (expense) and other income (expense), net	(772)	(1,174)	(13)	(1,514)	(2,507)	(2,646)	(3,168)	(3,307)	(2,788)	(12,977)	(19,829)
Income (loss) from continuing operations before income taxes	2,525	2,618	5,084	1,307	1,278	8,178	8,295	6,469	11,534	24,220	(7,973)
Income tax provision (benefit)	(12,350)	563	617	230	(1,326)	851	1,363	1,656	(10,940)	2,544	11,952
Income from continuing operations	14,875	2,055	4,467	1,077	2,604	7,327	6,932	4,813	22,474	21,676	(19,925)
Income (loss) from discontinued operations, net of tax.	(1,317)	(4,570)	414	322	1,244	1,489	3,143	1,449	(5,151)	7,325	19,286
Gain on disposal of discontinued operations, net of tax	2,255								2,255
Consolidated net income	15,813	(2,515)	4,881	1,399	3,848	8,816	10,075	6,262	19,578	29,001	(639)
Net (income) loss attributable to noncontrolling interest	5	(19)	(8)	(18)	7	29	(9)	(55)	(40)	(28)	(48)
Net income (loss) attributable to GSI Group Inc	$ 15,818	$ (2,534)	$ 4,873	$ 1,381	$ 3,855	$ 8,845	$ 10,066	$ 6,207	$ 19,538	$ 28,973	$ (687)
Earnings from continuing operations per common share:
Basic	$ 0.44	$ 0.06	$ 0.13	$ 0.03	$ 0.08	$ 0.22	$ 0.21	$ 0.14	$ 0.66	$ 0.65	$ (0.84)
Diluted	$ 0.44	$ 0.06	$ 0.13	$ 0.03	$ 0.08	$ 0.22	$ 0.21	$ 0.14	$ 0.66	$ 0.64	$ (0.84)
Earnings (loss) from discontinued operations per common share:
Basic	$ 0.03	$ (0.13)	$ 0.01	$ 0.01	$ 0.04	$ 0.04	$ 0.09	$ 0.05	$ (0.08)	$ 0.22	$ 0.81
Diluted	$ 0.03	$ (0.13)	$ 0.01	$ 0.01	$ 0.03	$ 0.04	$ 0.09	$ 0.05	$ (0.08)	$ 0.22	$ 0.81
Net income (loss) per common share:
Basic	$ 0.47	$ (0.07)	$ 0.14	$ 0.04	$ 0.12	$ 0.26	$ 0.30	$ 0.19	$ 0.58	$ 0.87	$ (0.03)
Diluted	$ 0.47	$ (0.07)	$ 0.14	$ 0.04	$ 0.11	$ 0.26	$ 0.30	$ 0.19	$ 0.58	$ 0.86	$ (0.03)