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CREDIT FACILITY AND INTEREST RATE SWAPS (Narrative) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Line Of Credit Facility [Line Items]        
Revolving credit facility, initiation date (Date) Nov. 08, 2012      
Revolving credit facility, borrowing capacity, maximum $ 40,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity      
Revolving credit facility, amount outstanding 0us-gaap_LinesOfCreditCurrent 0us-gaap_LinesOfCreditCurrent    
Revolving credit facility, expiration date (Date) Nov. 08, 2015      
Revolving credit facility, interest on borrowed funds 0us-gaap_InterestExpenseDebt 0us-gaap_InterestExpenseDebt 764us-gaap_InterestExpenseDebt  
Revolving credit facility, commitment fee, classified within interest expense, net 135us-gaap_LineOfCreditFacilityCommitmentFeeAmount 140us-gaap_LineOfCreditFacilityCommitmentFeeAmount 159us-gaap_LineOfCreditFacilityCommitmentFeeAmount  
Revolving credit facility, unamortized debt issuance costs 101us-gaap_DeferredFinanceCostsNet      
Derivative Line [Items]        
Floating rate debt hedged under interest rate Swap agreements     20,000us-gaap_DerivativeAmountOfHedgedItem  
Face amount of credit facility terminated with termination and expensing of two interest rate swaps       100,000us-gaap_DebtInstrumentFaceAmount
Interest rate swap agreements 0us-gaap_DerivativeNumberOfInstrumentsHeld 0us-gaap_DerivativeNumberOfInstrumentsHeld    
Forward-starting Swap 1 [Member]        
Derivative Line [Items]        
Inception date of Swap agreements (Date)     Jan. 31, 2012  
Interest rate swap, notional value     10,000us-gaap_NotionalAmountOfInterestRateCashFlowHedgeDerivatives
/ us-gaap_DerivativeByNatureAxis
= ntri_SwapContract1Member		  
Forward-starting Swap 2 [Member]        
Derivative Line [Items]        
Inception date of Swap agreements (Date)     Jun. 30, 2012  
Interest rate swap, notional value     10,000us-gaap_NotionalAmountOfInterestRateCashFlowHedgeDerivatives
/ us-gaap_DerivativeByNatureAxis
= ntri_SwapContract2Member		  
Floating to Fixed Swap 1 [Member]        
Derivative Line [Items]        
Interest rate swap, notional value     10,000us-gaap_NotionalAmountOfInterestRateCashFlowHedgeDerivatives
/ us-gaap_DerivativeByNatureAxis
= ntri_SwapContract3Member		  
Maturity date of Swaps (Date)     Aug. 03, 2012  
Floating to Fixed Swap 2 [Member]        
Derivative Line [Items]        
Interest rate swap, notional value     $ 10,000us-gaap_NotionalAmountOfInterestRateCashFlowHedgeDerivatives
/ us-gaap_DerivativeByNatureAxis
= ntri_SwapContract4Member		  
Maturity date of Swaps (Date)     Sep. 28, 2012  
Minimum [Member]        
Line Of Credit Facility [Line Items]        
Revolving credit facility, unused line fee annual percentage (in Percent) 0.25%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		      
Maximum [Member]        
Line Of Credit Facility [Line Items]        
Revolving credit facility, unused line fee annual percentage (in Percent) 0.375%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Base Rate [Member]        
Line Of Credit Facility [Line Items]        
Revolving credit facility, amount over Federal Funds Rate of Base Rate interest if this amount is higher than other base rate options (in Percent) 0.50%ntri_RevolvingCreditFacilityAmountOverFederalFundsRateOfBaseRateInterestIfThisAmountIsHigherThanOtherBaseRateOptions
/ us-gaap_VariableRateAxis
= us-gaap_BaseRateMember		      
Revolving credit facility, amount over LIBOR of Base Rate interest if this amount is higher than other base rate options (in Percent) 1.50%ntri_RevolvingCreditFacilityAmountOverLIBORRateOfBaseRateInterestIfThisAmountIsHigherThanOtherBaseRateOptions
/ us-gaap_VariableRateAxis
= us-gaap_BaseRateMember		      
Revolving credit facility interest rate (Description) Highest of Administrative Agent prime rate or 0.50% above Federal Funds Rate or the LIBOR rate for deposits in dollars for a one-month interest period as determined three business days prior to such date, plus 1.50%      
Base Rate [Member] | Minimum [Member]        
Line Of Credit Facility [Line Items]        
Revolving credit facility, basis spread on variable rate (in Percent) 0.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_VariableRateAxis
= us-gaap_BaseRateMember		      
Base Rate [Member] | Maximum [Member]        
Line Of Credit Facility [Line Items]        
Revolving credit facility, basis spread on variable rate (in Percent) 1.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableRateAxis
= us-gaap_BaseRateMember		      
LIBOR [Member]        
Line Of Credit Facility [Line Items]        
Revolving credit facility interest rate (Description) London Inter-Bank Offer Rate for the relevant term      
LIBOR [Member] | Minimum [Member]        
Line Of Credit Facility [Line Items]        
Revolving credit facility, basis spread on variable rate (in Percent) 1.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember		      
LIBOR [Member] | Maximum [Member]        
Line Of Credit Facility [Line Items]        
Revolving credit facility, basis spread on variable rate (in Percent) 2.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember