EMPLOYEE BENEFIT PLAN (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Percent contribution made by employer for the first 3 percent contribution made by employee (in Percent)	100.00%
Percent contribution made by employee matched at 100 percent by the employer (in Percent)	3.00%
Percent contribution made by employer for next 2 percent contribution made by employee (in Percent)	50.00%
Percent contribution matched at 50 by employer after initial 3 percent contribution (in Percent)	2.00%
Company's contributions during the period	$ 965	$ 892	$ 794

The percent contribution made by the employee which is matched at 100 percent by the employer.

No definition available.

Name:	ntri_DefinedContributionPlanPercentageContributionMatchedByEmployerBy100Percent
Namespace Prefix:	ntri_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The percent contribution made by the employee after initial 3 percent contribution which is matched at 50 percent by the employer.

No definition available.

Name:	ntri_DefinedContributionPlanPercentageContributionMatchedByEmployerBy50Percent
Namespace Prefix:	ntri_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The percentage contribution made by the employer for the first 3 percent contributed by employee.

No definition available.

Name:	ntri_DefinedContributionPlanPercentageMatchedByEmployerFirst3PercentContribution
Namespace Prefix:	ntri_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The percentage contribution made by the employer for the next 2 percent contributed by employee after initial 3 percent.

No definition available.

Name:	ntri_DefinedContributionPlanPercentageMatchedByEmployerNext2PercentContribution
Namespace Prefix:	ntri_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

No definition available.

Name:	us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of discretionary contributions made by an employer to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration