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Stock-Based Compensation (Narrative) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended
Mar. 28, 2015
Mar. 29, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restructuring charges $ 500us-gaap_RestructuringCharges $ 1,997us-gaap_RestructuringCharges
Employee stock purchase plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Purchase price of common stock, percent of market value 85.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember		  
Stock issued during the period 302,705us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember		 320,919us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
Stock-based Compensation    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restructuring charges $ 500us-gaap_RestructuringCharges
/ us-gaap_RestructuringCostAndReserveAxis
= form_ShareBasedCompensationExpenseMember