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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Summary Of Significant Accounting Policies [Abstract]      
Percentage of parent and wholly-owned subsidiaries assets and liabilities included in financial statements (in Percent) 100.00%ntri_PercentageOfAssetsAndLiabilitiesIncludedOfParentAndWhollyOwnedSubsidiariesInFinancialStatements    
Property, Plant, and Equipment [Line Items]      
Net book value of capitalized software $ 13,578us-gaap_CapitalizedComputerSoftwareNet   $ 13,162us-gaap_CapitalizedComputerSoftwareNet
Provision for estimated returns 3,979us-gaap_SalesReturnsGoods 3,909us-gaap_SalesReturnsGoods  
Valuation And Qualifying Accounts Disclosure [Line Items]      
Revenue from shipping and handling charges 716us-gaap_ShippingAndHandlingRevenue 605us-gaap_ShippingAndHandlingRevenue  
Concentration Risk [Line Items]      
Number of suppliers providing volume purchase rebates (in Integer) 1ntri_NumberOfSuppliersProvidingForVolumePurchaseRebates    
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Common stock equivalents excluded from weighted average shares outstanding for diluted income per common share purposes (in Shares) 314us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 355us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount  
Payment for income taxes 1,902us-gaap_IncomeTaxesPaid 18us-gaap_IncomeTaxesPaid  
Payment for interest 71us-gaap_InterestPaid 59us-gaap_InterestPaid  
Non-cash capital additions 140us-gaap_CapitalExpendituresIncurredButNotYetPaid 35us-gaap_CapitalExpendituresIncurredButNotYetPaid  
Reduced cost of revenue, rebate earned 416us-gaap_RecoveryOfDirectCosts 276us-gaap_RecoveryOfDirectCosts  
Amount of rebate receivable recorded in balance sheet 775ntri_VendorRebatesReceivableRecordedInAccountsReceivable   360ntri_VendorRebatesReceivableRecordedInAccountsReceivable
Level 2 [Member]      
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value, financial instruments 0us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
  0us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Level 3 [Member]      
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value, financial instruments 0us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
  0us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Major Supplier 1 [Member] | Inventory Purchases [Member]      
Concentration Risk [Line Items]      
Percent concentration risk (in Percent) 19.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntri_InventoryPurchasesMember
/ us-gaap_ConcentrationRiskByTypeAxis
= ntri_MajorSupplier1Member
14.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntri_InventoryPurchasesMember
/ us-gaap_ConcentrationRiskByTypeAxis
= ntri_MajorSupplier1Member
 
Major Supplier 2 [Member] | Inventory Purchases [Member]      
Concentration Risk [Line Items]      
Percent concentration risk (in Percent) 18.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntri_InventoryPurchasesMember
/ us-gaap_ConcentrationRiskByTypeAxis
= ntri_MajorSupplier2Member
11.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntri_InventoryPurchasesMember
/ us-gaap_ConcentrationRiskByTypeAxis
= ntri_MajorSupplier2Member
 
Fulfillment Provider [Member] | Fulfillment Cost [Member]      
Concentration Risk [Line Items]      
Percent concentration risk (in Percent) 100.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntri_FulfillmentCostMember
/ us-gaap_ConcentrationRiskByTypeAxis
= ntri_FulfillmentProviderMember
   
Allowance for Sales Returns [Member]      
Valuation And Qualifying Accounts Disclosure [Line Items]      
Returns reserve balance $ 1,376us-gaap_ValuationAllowancesAndReservesBalance
/ us-gaap_ValuationAllowancesAndReservesTypeAxis
= us-gaap_AllowanceForSalesReturnsMember
  $ 762us-gaap_ValuationAllowancesAndReservesBalance
/ us-gaap_ValuationAllowancesAndReservesTypeAxis
= us-gaap_AllowanceForSalesReturnsMember
Minimum [Member] | Shipping Provider [Member] | Shipping Costs [Member]      
Concentration Risk [Line Items]      
Percent concentration risk (in Percent) 94.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= ntri_ShippingCostsMember
/ us-gaap_ConcentrationRiskByTypeAxis
= ntri_ShippingProviderMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Internal-Use Software and Website Development [Member] | Minimum [Member]      
Property, Plant, and Equipment [Line Items]      
Estimated useful life (in Duration) 2 years    
Internal-Use Software and Website Development [Member] | Maximum [Member]      
Property, Plant, and Equipment [Line Items]      
Estimated useful life (in Duration) 5 years    
Fixed Assets [Member] | Minimum [Member]      
Property, Plant, and Equipment [Line Items]      
Estimated useful life (in Duration) 2 years    
Fixed Assets [Member] | Maximum [Member]      
Property, Plant, and Equipment [Line Items]      
Estimated useful life (in Duration) 7 years