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EQUITY INSTRUMENTS (Narrative) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Recognized increase in taxes payable from the exercise of certain stock options and restricted stock $ 68ntri_ShareBasedCompensationTaxesPayableIncreaseDecrease $ 203ntri_ShareBasedCompensationTaxesPayableIncreaseDecrease $ 593ntri_ShareBasedCompensationTaxesPayableIncreaseDecrease
Total unrecognized compensation expense related to unvested share-based compensation arrangements 8,832us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Period over which unvested share-based compensation is expected to be recognized, weighted average (in Duration) 1 year 3 months 18 days    
Share Based Goods And Non-employee Services Transaction [Line Items]      
Common stock issued for services (in Shares) 29us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued 50us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued 65us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
Board of Directors Compensation [Member]      
Share Based Goods And Non-employee Services Transaction [Line Items]      
Common stock issued for services (in Shares) 27us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_DirectorMember
50us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_DirectorMember
43us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_DirectorMember
Common stock issued for services, costs recognized 455us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionExpense
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_DirectorMember
455us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionExpense
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_DirectorMember
455us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionExpense
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_DirectorMember
Non-Employee Services [Member]      
Share Based Goods And Non-employee Services Transaction [Line Items]      
Common stock issued for services (in Shares) 2us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_GoodsAndServicesExchangedForEquityInstrumentMember
0us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_GoodsAndServicesExchangedForEquityInstrumentMember
22us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionQuantityOfSecuritiesIssued
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_GoodsAndServicesExchangedForEquityInstrumentMember
Common stock issued for services, costs recognized 30us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionExpense
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_GoodsAndServicesExchangedForEquityInstrumentMember
195us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionExpense
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_GoodsAndServicesExchangedForEquityInstrumentMember
573us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionExpense
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= us-gaap_GoodsAndServicesExchangedForEquityInstrumentMember
Stock Option [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Compensation expense for stock option awards 426us-gaap_StockOptionPlanExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
382us-gaap_StockOptionPlanExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
1,163us-gaap_StockOptionPlanExpense
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Weighted-average grant date fair value of stock options granted during the period (in Dollars per Share) $ 3.88us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
$ 1.61us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
$ 1.96us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Stock options exercised during the period, total intrinsic value 219us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
67us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_StockOptionMember
Restricted Stock [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Compensation expense, issuance of restricted stock and restricted stock units $ 4,740us-gaap_RestrictedStockExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
$ 4,800us-gaap_RestrictedStockExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
$ 7,721us-gaap_RestrictedStockExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Number of shares expected to vest (in Shares) 403ntri_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
   
Restricted Stock Units [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Minimum percentage that restricted stock performance based units may increase over original award caused by achievement of goals (in Percent) 0.00%ntri_MinimumPercentageIncreaseOfPerformanceBasedUnitsVesting
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
   
Maximum percentage that restricted stock performance based units may increase over original award caused by achievement of goals (in Percent) 150.00%ntri_MaximumPercentageIncreaseOfPerformanceBasedUnitsVesting
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
   
Minimum percentage of awards that employees are entitled to vest based on market price conditions (in Percent) 0.00%ntri_MinimumPercentageOfAwardsBasedOnMarketBasedConditions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
   
Maximum percentage of awards that employees are entitled to vest based on market based conditions (in Percent) 200.00%ntri_MaximumPercentageOfAwardsBasedOnMarketBasedConditions
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
   
Performance price measurement period (in Duration) 20 days    
Number of shares expected to vest (in Shares) 355ntri_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExpectedToVestNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
   
Maximum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Award expiration period (in Duration) 10 years    
Maximum [Member] | Stock Option [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Award expiration period (in Duration) 10 years    
Maximum [Member] | Restricted Stock [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Award vesting period (in Duration) 4 years    
Minimum [Member] | Stock Option [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Award expiration period (in Duration) 7 years    
Minimum [Member] | Restricted Stock [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Award vesting period (in Duration) 2 years    
1999 Equity Incentive Plan [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Shares authorized under an equity incentive plan (in Shares) 1,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= ntri_EquityIncentivePlan1999Member
   
Awards exercisable after 10 years from date award is granted (in Shares) 0ntri_NumberOfAwardsExercisableAfter10YearsFromDateAwardIsGranted
/ us-gaap_PlanNameAxis
= ntri_EquityIncentivePlan1999Member
   
2000 Equity Incentive Plan [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Shares authorized under an equity incentive plan (in Shares) 5,600us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= ntri_EquityIncentivePlan2000Member
   
Awards exercisable after 10 years from date award is granted (in Shares) 0ntri_NumberOfAwardsExercisableAfter10YearsFromDateAwardIsGranted
/ us-gaap_PlanNameAxis
= ntri_EquityIncentivePlan2000Member
   
Amended and Restated 2008 Long Term Incentive Plan [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Shares authorized under an equity incentive plan (in Shares) 5,400us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= ntri_AmendedAndRestated2008LongTermIncentivePlanMember
   
Shares available for grant (in Shares) 1,776us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= ntri_AmendedAndRestated2008LongTermIncentivePlanMember
   
Awards exercisable after 10 years from date award is granted (in Shares) 0ntri_NumberOfAwardsExercisableAfter10YearsFromDateAwardIsGranted
/ us-gaap_PlanNameAxis
= ntri_AmendedAndRestated2008LongTermIncentivePlanMember