CONSOLIDATED STATEMENTS OF OPERATIONS (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Sales	$ 364,706us-gaap_SalesRevenueNet	$ 316,910us-gaap_SalesRevenueNet	$ 243,796us-gaap_SalesRevenueNet
Cost of sales	214,539us-gaap_CostOfGoodsSold	184,683us-gaap_CostOfGoodsSold	138,278us-gaap_CostOfGoodsSold
Gross profit	150,167us-gaap_GrossProfit	132,227us-gaap_GrossProfit	105,518us-gaap_GrossProfit
Operating expenses:
Research and development and engineering	28,954us-gaap_ResearchAndDevelopmentExpense	23,787us-gaap_ResearchAndDevelopmentExpense	18,530us-gaap_ResearchAndDevelopmentExpense
Selling, general and administrative	84,380	76,337	59,707
Amortization of purchased intangible assets	10,262us-gaap_AmortizationOfIntangibleAssets	7,270us-gaap_AmortizationOfIntangibleAssets	2,650us-gaap_AmortizationOfIntangibleAssets
Restructuring and acquisition related costs	1,935	5,387	4,369
Impairment of goodwill and intangible assets	41,442	0	0
Total operating expenses	166,973us-gaap_OperatingExpenses	112,781us-gaap_OperatingExpenses	85,256us-gaap_OperatingExpenses
Operating income (loss) from continuing operations	(16,806)us-gaap_OperatingIncomeLoss	19,446us-gaap_OperatingIncomeLoss	20,262us-gaap_OperatingIncomeLoss
Interest income (expense), net	(5,096)us-gaap_InterestIncomeExpenseNet	(3,468)us-gaap_InterestIncomeExpenseNet	(2,792)us-gaap_InterestIncomeExpenseNet
Foreign exchange transaction gains (losses), net	1,281	(1,301)	(1,347)
Other income (expense), net	2,706us-gaap_OtherNonoperatingIncomeExpense	1,500us-gaap_OtherNonoperatingIncomeExpense	579us-gaap_OtherNonoperatingIncomeExpense
Income (loss) from continuing operations before income taxes	(17,915)	16,177	16,702
Income tax provision (benefit)	(1,006)us-gaap_IncomeTaxExpenseBenefit	6,200us-gaap_IncomeTaxExpenseBenefit	(11,595)us-gaap_IncomeTaxExpenseBenefit
Income (loss) from continuing operations	(16,909)	9,977	28,297
Loss from discontinued operations, net of tax	(5,607)	(2,054)	(10,974)
Gain (loss) on disposal of discontinued operations, net of tax	(1,726)	(592)	2,255
Consolidated net income (loss)	(24,242)us-gaap_ProfitLoss	7,331us-gaap_ProfitLoss	19,578us-gaap_ProfitLoss
Less: Net income attributable to noncontrolling interest	(10)	(22)	(40)
Net income (loss) attributable to GSI Group Inc.	(24,252)us-gaap_NetIncomeLoss	7,309us-gaap_NetIncomeLoss	19,538us-gaap_NetIncomeLoss
Earnings (loss) per common share from continuing operations:
Basic	$ (0.49)	$ 0.29	$ 0.84
Diluted	$ (0.49)	$ 0.29	$ 0.84
Loss per common share from discontinued operations:
Basic	$ (0.21)	$ (0.08)	$ (0.26)
Diluted	$ (0.21)	$ (0.08)	$ (0.26)
Earnings (loss) per common share attributable to GSI Group Inc.:
Basic	$ (0.70)us-gaap_EarningsPerShareBasic	$ 0.21us-gaap_EarningsPerShareBasic	$ 0.58us-gaap_EarningsPerShareBasic
Diluted	$ (0.70)us-gaap_EarningsPerShareDiluted	$ 0.21us-gaap_EarningsPerShareDiluted	$ 0.58us-gaap_EarningsPerShareDiluted
Weighted average common shares outstanding—basic	34,352	34,073	33,775
Weighted average common shares outstanding—diluted	34,352	34,396	33,936
Amounts attributable to GSI Group Inc.:
Income (loss) from continuing operations	(16,909)	9,977	28,297
Loss from discontinued operations	(7,343)	(2,668)	(8,759)
Net income (loss) attributable to GSI Group Inc.	$ (24,252)us-gaap_NetIncomeLoss	$ 7,309us-gaap_NetIncomeLoss	$ 19,538us-gaap_NetIncomeLoss

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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.