REVENUE - U.S. and Canada Revenue (Details) - USD ($)	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue	$ 617,944,000	$ 650,238,000		$ 1,225,477,000	$ 1,305,186,000
Operating Segment
Disaggregation of Revenue [Line Items]
Revenue	683,182,000	683,624,000		1,355,378,000	1,377,128,000
Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	517,317,000	527,814,000		1,029,731,000	1,064,435,000
Product Sales | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	397,090,000	407,158,000		794,869,000	809,316,000
Protein | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	87,072,000	90,706,000		174,742,000	182,969,000
Performance supplements | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	73,100,000	74,709,000		148,716,000	148,017,000
Weight management | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	38,686,000	39,758,000		78,473,000	80,275,000
Weight management | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	49,495,000	52,112,000		99,866,000	103,348,000
Herbs / Greens | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	16,945,000	17,018,000		33,103,000	32,735,000
Wellness | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	49,680,000	51,175,000		97,381,000	99,596,000
Health / Beauty | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	47,525,000	48,887,000		95,579,000	97,597,000
Food / Drink | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	28,709,000	25,448,000		54,069,000	49,570,000
General merchandise | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	5,878,000	7,345,000		12,940,000	15,209,000
Wholesale sales to franchisees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	60,675,000	66,082,000		117,835,000	130,363,000
Royalties and franchise fees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	8,532,000	9,492,000		17,280,000	18,823,000
Sublease income | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	11,633,000	12,362,000		23,398,000	24,958,000
Cooperative advertising and other franchise support fees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	5,973,000	6,349,000		$ 11,506,000	12,390,000
Gold Card revenue recognized in U.S. | U.S. and Canada
Disaggregation of Revenue [Line Items]
Deferred revenue						$ 24,400,000
Recognition of deferred revenue			$ 1,400,000
Period for loyalty rewards				1 year
Annual membership fee per customer	39.99			$ 39.99
Gold Card revenue recognized in U.S. | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	0	0		0	24,399,000
Other | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	$ 33,414,000	$ 26,371,000		$ 64,843,000	$ 44,186,000