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Income Taxes (Discussion of Valuation Allowance and Net Deferred Tax Assets) (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2011
Jan. 03, 2015
Dec. 28, 2013
Valuation Allowance [Line Items]      
Tax provision related to the intercompany sale of inventory     $ 13,700,000lscc_IntercompanySaleOfInventoryAndFixedAssetsTaxExpenseBenefit
Resulting tax benefit from realization of the benefit of a portion of remaining deferred tax assets 11,500,000lscc_RealizationOfDeferredTaxAssetIncomeTaxBenefit    
Resulting net deferred tax asset 21,300,000lscc_DeferredTaxAssetOtherNetOfValuationAllowance    
Increase (decrease) in valuation allowance   9,700,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount  
Deferred tax assets:      
Accrued expenses and reserves   5,416,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities 4,959,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAccruedLiabilities
Stock-based and deferred compensation   5,530,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits 4,986,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits
Intangible assets   9,841,000us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets 8,456,000us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets
Fixed assets   983,000us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment 828,000us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment
Net operating loss carry forwards   96,543,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards 101,144,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards
Tax credit carry forwards   40,588,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwards 36,644,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwards
Capital loss carry forwards   4,142,000us-gaap_DeferredTaxAssetsCapitalLossCarryforwards 6,698,000us-gaap_DeferredTaxAssetsCapitalLossCarryforwards
Unrealized loss on securities   0us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross 758,000us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross
Other   220,000us-gaap_DeferredTaxAssetsOther 143,000us-gaap_DeferredTaxAssetsOther
Gross deferred tax assets   163,263,000us-gaap_DeferredTaxAssetsGross 164,616,000us-gaap_DeferredTaxAssetsGross
Less: valuation allowance   (141,215,000)us-gaap_DeferredTaxAssetsValuationAllowance (150,528,000)us-gaap_DeferredTaxAssetsValuationAllowance
Net deferred tax assets   22,048,000us-gaap_DeferredTaxAssetsNet 14,088,000us-gaap_DeferredTaxAssetsNet
Deferred tax liabilities      
Other   717,000us-gaap_DeferredTaxLiabilitiesOther 969,000us-gaap_DeferredTaxLiabilitiesOther
Total deferred tax liabilities   717,000us-gaap_DeferredIncomeTaxLiabilities 969,000us-gaap_DeferredIncomeTaxLiabilities
Net deferred tax assets   21,331,000us-gaap_DeferredTaxAssetsLiabilitiesNet 13,119,000us-gaap_DeferredTaxAssetsLiabilitiesNet
Do not expire      
Valuation Allowance [Line Items]      
Tax credit carryforwards   24,500,000us-gaap_TaxCreditCarryforwardAmount
/ lscc_ExpirationPeriodAxis
= lscc_NoExpirationDateMember		  
Prepaid expenses and other current assets      
Valuation Allowance [Line Items]      
Net deferred tax assets, current   1,200,000us-gaap_DeferredTaxAssetsNetCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember		 1,400,000us-gaap_DeferredTaxAssetsNetCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_PrepaidExpensesAndOtherCurrentAssetsMember
Federal      
Valuation Allowance [Line Items]      
Operating loss carryforwards   301,800,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		  
Tax credit carryforwards   17,700,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		  
State      
Valuation Allowance [Line Items]      
Operating loss carryforwards   141,300,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		  
Tax credit carryforwards   $ 26,300,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember