CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Apr. 03, 2015	Mar. 28, 2014
Income Statement [Abstract]
Sales	$ 94,614us-gaap_SalesRevenueNet	$ 79,133us-gaap_SalesRevenueNet
Cost of sales	54,608us-gaap_CostOfGoodsSold	47,028us-gaap_CostOfGoodsSold
Gross profit	40,006us-gaap_GrossProfit	32,105us-gaap_GrossProfit
Operating expenses:
Research and development and engineering	8,215us-gaap_ResearchAndDevelopmentExpense	5,857us-gaap_ResearchAndDevelopmentExpense
Selling, general and administrative	22,068	19,618
Amortization of purchased intangible assets	1,889us-gaap_AmortizationOfIntangibleAssets	1,744us-gaap_AmortizationOfIntangibleAssets
Restructuring, acquisition and divestiture related costs	2,437	818
Total operating expenses	34,609us-gaap_OperatingExpenses	28,037us-gaap_OperatingExpenses
Operating income from continuing operations	5,397us-gaap_OperatingIncomeLoss	4,068us-gaap_OperatingIncomeLoss
Interest income (expense), net	(1,397)us-gaap_InterestIncomeExpenseNet	(837)us-gaap_InterestIncomeExpenseNet
Foreign exchange transaction gains (losses), net	517	(19)
Other income (expense), net	729us-gaap_OtherNonoperatingIncomeExpense	581us-gaap_OtherNonoperatingIncomeExpense
Income from continuing operations before income taxes	5,246	3,793
Income tax provision	1,800us-gaap_IncomeTaxExpenseBenefit	937us-gaap_IncomeTaxExpenseBenefit
Income from continuing operations	3,446	2,856
Loss from discontinued operations, net of tax		(1,866)
Consolidated net income	3,446us-gaap_ProfitLoss	990us-gaap_ProfitLoss
Less: Net income attributable to noncontrolling interest		(7)
Net income attributable to GSI Group Inc.	$ 3,446us-gaap_NetIncomeLoss	$ 983us-gaap_NetIncomeLoss
Earnings per common share from continuing operations:
Basic	$ 0.10	$ 0.08
Diluted	$ 0.10	$ 0.08
Loss per common share from discontinued operations:
Basic		$ (0.05)
Diluted		$ (0.05)
Earnings per common share attributable to GSI Group Inc.:
Basic	$ 0.10us-gaap_EarningsPerShareBasic	$ 0.03us-gaap_EarningsPerShareBasic
Diluted	$ 0.10us-gaap_EarningsPerShareDiluted	$ 0.03us-gaap_EarningsPerShareDiluted
Weighted average common shares outstanding—basic	34,506	34,227
Weighted average common shares outstanding—diluted	34,999	34,669

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Balance Type:	na
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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.