BACKGROUND (Narrative) (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Background [Abstract]
Years in business the company has exceeded (in Duration)	45 years
Percentage of revenues generated in the United States during the year (in Percent)	99.00%	99.00%	99.00%

Percentage of revenues from external customers attributed to the entity's country of domicile.

No definition available.

Name:	ntri_EntityWideDisclosureOnGeographicAreasRevenuePercentageFromExternalCustomersAttributedToEntitysCountryOfDomicile
Namespace Prefix:	ntri_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration