REVENUE - U.S. and Canada Revenue (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue	$ 580,185,000	$ 612,953,000		$ 1,805,662,000	$ 1,918,139,000
Operating Segment
Disaggregation of Revenue [Line Items]
Revenue	643,880,000	650,164,000		1,999,258,000	2,027,292,000
Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	476,519,000	492,383,000		1,506,250,000	1,556,818,000
Product Sales | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	362,855,000	379,771,000		1,157,724,000	1,189,087,000
Protein | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	76,738,000	83,652,000		251,480,000	266,621,000
Performance supplements | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	68,809,000	69,770,000		217,525,000	217,787,000
Weight management | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	29,575,000	32,622,000		108,048,000	112,897,000
Vitamins | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	48,322,000	51,015,000		148,188,000	154,363,000
Herbs / Greens | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	15,872,000	16,817,000		48,975,000	49,552,000
Wellness | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	46,245,000	47,888,000		143,626,000	147,484,000
Health / Beauty | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	43,332,000	48,027,000		138,911,000	145,624,000
Food / Drink | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	28,325,000	23,248,000		82,394,000	72,818,000
General merchandise | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	5,637,000	6,732,000		18,577,000	21,941,000
Wholesale sales to franchisees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	58,199,000	59,413,000		176,034,000	189,776,000
Royalties and franchise fees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	7,939,000	8,649,000		25,219,000	27,472,000
Sublease income | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	11,087,000	12,170,000		34,485,000	37,128,000
Cooperative advertising and other franchise support fees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue	4,739,000	5,769,000		$ 16,245,000	18,159,000
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	$ 31,700,000	$ 26,611,000		$ 96,543,000	$ 70,797,000