REVENUE - U.S. and Canada Revenue (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue	$ 470,351	$ 499,076	$ 533,997	$ 564,764	$ 547,861	$ 580,185	$ 617,944	$ 607,533		$ 2,068,188	$ 2,353,523	$ 2,480,962
Deferred revenue									$ 24,400
Recognition of deferred revenue									$ 1,400
Operating Segment
Disaggregation of Revenue [Line Items]
Revenue										2,103,693	2,617,734	2,646,275
Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										1,822,327	1,951,220	2,018,931
Product Sales | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										1,397,782	1,498,480	1,541,586
Protein | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										295,135	320,751	338,773
Performance supplements | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										283,473	280,835	281,532
Weight management | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										100,356	128,723	140,148
Vitamins | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										180,742	195,853	203,569
Herbs / Greens | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										59,578	66,025	66,324
Wellness | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										179,059	191,995	196,942
Health / Beauty | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										179,015	181,185	190,977
Food / Drink | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										98,134	109,094	94,390
General merchandise | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										22,290	24,019	28,931
Wholesale sales to franchisees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										219,644	225,106	242,521
Royalties and franchise fees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										31,527	32,733	35,212
Sublease income | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										42,282	45,506	48,972
Cooperative advertising and other franchise support fees | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										18,530	20,815	23,424
Gold Card revenue recognized in U.S. | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										0	0	24,399
Other | Operating Segment | U.S. and Canada
Disaggregation of Revenue [Line Items]
Revenue										$ 112,562	$ 128,580	$ 102,817