Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 26, 2014	Jun. 27, 2014	Mar. 28, 2014	Dec. 31, 2013	Sep. 27, 2013	Jun. 28, 2013	Mar. 29, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 94,012us-gaap_SalesRevenueNet	$ 94,656us-gaap_SalesRevenueNet	$ 96,905us-gaap_SalesRevenueNet	$ 79,133us-gaap_SalesRevenueNet	$ 82,212us-gaap_SalesRevenueNet	$ 79,858us-gaap_SalesRevenueNet	$ 79,769us-gaap_SalesRevenueNet	$ 75,071us-gaap_SalesRevenueNet	$ 364,706us-gaap_SalesRevenueNet	$ 316,910us-gaap_SalesRevenueNet	$ 243,796us-gaap_SalesRevenueNet
Cost of goods sold	54,284us-gaap_CostOfGoodsSold	54,973us-gaap_CostOfGoodsSold	58,254us-gaap_CostOfGoodsSold	47,028us-gaap_CostOfGoodsSold	48,016us-gaap_CostOfGoodsSold	45,697us-gaap_CostOfGoodsSold	46,530us-gaap_CostOfGoodsSold	44,440us-gaap_CostOfGoodsSold	214,539us-gaap_CostOfGoodsSold	184,683us-gaap_CostOfGoodsSold	138,278us-gaap_CostOfGoodsSold
Gross profit	39,728us-gaap_GrossProfit	39,683us-gaap_GrossProfit	38,651us-gaap_GrossProfit	32,105us-gaap_GrossProfit	34,196us-gaap_GrossProfit	34,161us-gaap_GrossProfit	33,239us-gaap_GrossProfit	30,631us-gaap_GrossProfit	150,167us-gaap_GrossProfit	132,227us-gaap_GrossProfit	105,518us-gaap_GrossProfit
Operating expenses:
Research and development and engineering	7,837us-gaap_ResearchAndDevelopmentExpense	7,735us-gaap_ResearchAndDevelopmentExpense	7,525us-gaap_ResearchAndDevelopmentExpense	5,857us-gaap_ResearchAndDevelopmentExpense	5,825us-gaap_ResearchAndDevelopmentExpense	6,031us-gaap_ResearchAndDevelopmentExpense	6,115us-gaap_ResearchAndDevelopmentExpense	5,816us-gaap_ResearchAndDevelopmentExpense	28,954us-gaap_ResearchAndDevelopmentExpense	23,787us-gaap_ResearchAndDevelopmentExpense	18,530us-gaap_ResearchAndDevelopmentExpense
Selling, general and administrative	21,840	21,512	21,410	19,618	19,268	19,006	19,374	18,689	84,380	76,337	59,707
Amortization expense – operating expenses	2,799us-gaap_AmortizationOfIntangibleAssets	2,843us-gaap_AmortizationOfIntangibleAssets	2,876us-gaap_AmortizationOfIntangibleAssets	1,744us-gaap_AmortizationOfIntangibleAssets	1,645us-gaap_AmortizationOfIntangibleAssets	1,772us-gaap_AmortizationOfIntangibleAssets	1,617us-gaap_AmortizationOfIntangibleAssets	2,236us-gaap_AmortizationOfIntangibleAssets	10,262us-gaap_AmortizationOfIntangibleAssets	7,270us-gaap_AmortizationOfIntangibleAssets	2,650us-gaap_AmortizationOfIntangibleAssets
Restructuring and acquisition related costs	(14)	771	360	818	663	1,553	743	2,428	1,935	5,387	4,369
Impairment of goodwill and intangible assets	41,442								41,442	0	0
Total operating expenses	73,904us-gaap_OperatingExpenses	32,861us-gaap_OperatingExpenses	32,171us-gaap_OperatingExpenses	28,037us-gaap_OperatingExpenses	27,401us-gaap_OperatingExpenses	28,362us-gaap_OperatingExpenses	27,849us-gaap_OperatingExpenses	29,169us-gaap_OperatingExpenses	166,973us-gaap_OperatingExpenses	112,781us-gaap_OperatingExpenses	85,256us-gaap_OperatingExpenses
Operating income (loss) from continuing operations	(34,176)us-gaap_OperatingIncomeLoss	6,822us-gaap_OperatingIncomeLoss	6,480us-gaap_OperatingIncomeLoss	4,068us-gaap_OperatingIncomeLoss	6,795us-gaap_OperatingIncomeLoss	5,799us-gaap_OperatingIncomeLoss	5,390us-gaap_OperatingIncomeLoss	1,462us-gaap_OperatingIncomeLoss	(16,806)us-gaap_OperatingIncomeLoss	19,446us-gaap_OperatingIncomeLoss	20,262us-gaap_OperatingIncomeLoss
Interest income (expense), foreign exchange transaction gains (losses) and other income (expense), net	(127)us-gaap_NonoperatingIncomeExpense	310us-gaap_NonoperatingIncomeExpense	(1,017)us-gaap_NonoperatingIncomeExpense	(275)us-gaap_NonoperatingIncomeExpense	(850)us-gaap_NonoperatingIncomeExpense	(2,022)us-gaap_NonoperatingIncomeExpense	(1,087)us-gaap_NonoperatingIncomeExpense	690us-gaap_NonoperatingIncomeExpense
Income (loss) from continuing operations before income taxes	(34,303)	7,132	5,463	3,793	5,945	3,777	4,303	2,152	(17,915)	16,177	16,702
Income tax provision	(6,013)us-gaap_IncomeTaxExpenseBenefit	2,013us-gaap_IncomeTaxExpenseBenefit	2,057us-gaap_IncomeTaxExpenseBenefit	937us-gaap_IncomeTaxExpenseBenefit	1,246us-gaap_IncomeTaxExpenseBenefit	1,533us-gaap_IncomeTaxExpenseBenefit	3,018us-gaap_IncomeTaxExpenseBenefit	403us-gaap_IncomeTaxExpenseBenefit	(1,006)us-gaap_IncomeTaxExpenseBenefit	6,200us-gaap_IncomeTaxExpenseBenefit	(11,595)us-gaap_IncomeTaxExpenseBenefit
Income (loss) from continuing operations	(28,290)	5,119	3,406	2,856	4,699	2,244	1,285	1,749	(16,909)	9,977	28,297
Consolidated loss from discontinued operations	(790)	(273)	(2,678)	(1,866)	(411)	(185)	(1,827)	369	(7,333)	(2,646)	(8,719)
Gain (loss) on disposal of discontinued operations, net of tax	(1,405)	(321)				(281)	(311)		(1,726)	(592)	2,255
Consolidated net income (loss)	(30,485)us-gaap_ProfitLoss	4,525us-gaap_ProfitLoss	728us-gaap_ProfitLoss	990us-gaap_ProfitLoss	4,288us-gaap_ProfitLoss	1,778us-gaap_ProfitLoss	(853)us-gaap_ProfitLoss	2,118us-gaap_ProfitLoss	(24,242)us-gaap_ProfitLoss	7,331us-gaap_ProfitLoss	19,578us-gaap_ProfitLoss
Less: income attributable to noncontrolling interest			(3)	(7)	20	12	(18)	(36)	(10)	(22)	(40)
Net income (loss) attributable to GSI Group Inc.	$ (30,485)us-gaap_NetIncomeLoss	$ 4,525us-gaap_NetIncomeLoss	$ 725us-gaap_NetIncomeLoss	$ 983us-gaap_NetIncomeLoss	$ 4,308us-gaap_NetIncomeLoss	$ 1,790us-gaap_NetIncomeLoss	$ (871)us-gaap_NetIncomeLoss	$ 2,082us-gaap_NetIncomeLoss	$ (24,252)us-gaap_NetIncomeLoss	$ 7,309us-gaap_NetIncomeLoss	$ 19,538us-gaap_NetIncomeLoss
Earnings (loss) per common share from continuing operations:
Basic	$ (0.82)	$ 0.15	$ 0.10	$ 0.08	$ 0.14	$ 0.07	$ 0.03	$ 0.05	$ (0.49)	$ 0.29	$ 0.84
Diluted	$ (0.82)	$ 0.15	$ 0.10	$ 0.08	$ 0.14	$ 0.07	$ 0.03	$ 0.05	$ (0.49)	$ 0.29	$ 0.84
Earnings (loss) per common share from discontinued operations:
Basic	$ (0.06)	$ (0.02)	$ (0.08)	$ (0.05)	$ (0.01)	$ (0.02)	$ (0.06)	$ 0.01	$ (0.21)	$ (0.08)	$ (0.26)
Diluted	$ (0.06)	$ (0.02)	$ (0.08)	$ (0.05)	$ (0.01)	$ (0.02)	$ (0.06)	$ 0.01	$ (0.21)	$ (0.08)	$ (0.26)
Earnings (loss) per common share attributable to GSI Group Inc.:
Basic	$ (0.88)us-gaap_EarningsPerShareBasic	$ 0.13us-gaap_EarningsPerShareBasic	$ 0.02us-gaap_EarningsPerShareBasic	$ 0.03us-gaap_EarningsPerShareBasic	$ 0.13us-gaap_EarningsPerShareBasic	$ 0.05us-gaap_EarningsPerShareBasic	$ (0.03)us-gaap_EarningsPerShareBasic	$ 0.06us-gaap_EarningsPerShareBasic	$ (0.70)us-gaap_EarningsPerShareBasic	$ 0.21us-gaap_EarningsPerShareBasic	$ 0.58us-gaap_EarningsPerShareBasic
Diluted	$ (0.88)us-gaap_EarningsPerShareDiluted	$ 0.13us-gaap_EarningsPerShareDiluted	$ 0.02us-gaap_EarningsPerShareDiluted	$ 0.03us-gaap_EarningsPerShareDiluted	$ 0.13us-gaap_EarningsPerShareDiluted	$ 0.05us-gaap_EarningsPerShareDiluted	$ (0.03)us-gaap_EarningsPerShareDiluted	$ 0.06us-gaap_EarningsPerShareDiluted	$ (0.70)us-gaap_EarningsPerShareDiluted	$ 0.21us-gaap_EarningsPerShareDiluted	$ 0.58us-gaap_EarningsPerShareDiluted

Name:	gsig_EarningsPerShareFromContinuingOperationsAbstract
Namespace Prefix:	gsig_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This element represents (i) restructuring charges, which are amounts charged against earnings in the period for incurred and estimated costs associated with exit from or disposal of business activities or restructurings pursuant to a program that is planned and controlled by management, and materially changes either the scope of a business undertaken by an entity, or the manner in which that business is conducted; and (ii) acquisition-related costs, which are costs incurred to effect a business combination which costs have been expensed during the period. Such costs can include transaction costs such as banking, legal, accounting and other costs directly related to effect a business combination as well as external, incremental costs of integration planning that are directly related to a business combination, including costs associated with preparing for systems and other integration activities.