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BACKGROUND (Narrative) (Details)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Background [Abstract]      
Years in business the company has exceeded (in Duration) 40 years    
Percentage of revenues generated in the United States during the year (in Percent) 99.00%ntri_EntityWideDisclosureOnGeographicAreasRevenuePercentageFromExternalCustomersAttributedToEntitysCountryOfDomicile 99.00%ntri_EntityWideDisclosureOnGeographicAreasRevenuePercentageFromExternalCustomersAttributedToEntitysCountryOfDomicile 98.00%ntri_EntityWideDisclosureOnGeographicAreasRevenuePercentageFromExternalCustomersAttributedToEntitysCountryOfDomicile