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Segment and Geographic Data - Information About Reportable Segments (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Nov. 29, 2017	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Segment Reporting Information [Line Items]
Net revenue		$ 332,583	$ 348,567	$ 369,023	$ 363,164	$ 330,386	$ 365,765	$ 409,747	$ 408,223	$ 1,413,337	$ 1,514,121	$ 1,306,911
Operating income		65,886	94,729	105,473	21,897	80,347	118,860	127,708	62,073	287,985	388,985	267,305
Interest expense										135,267	142,346	112,784
Other expense, net										1,758	2,578	472
Early extinguishment of debt, net	$ 10,524									0	0	8,969
Provision for (benefit from) income taxes										31,513	20,493	(18,237)
Net income										119,447	223,568	163,317
Net loss attributable to the noncontrolling interest										169	181	197
Net income attributable to WW International, Inc.		$ 29,383	$ 47,086	$ 53,834	$ (10,687)	$ 43,785	$ 70,132	$ 70,720	$ 39,112	119,616	223,749	163,514
Depreciation and amortization										54,335	52,600	56,992
Operating Segments
Segment Reporting Information [Line Items]
Operating income										391,055	494,725	336,857
Depreciation and amortization										39,597	40,568	42,535
General corporate expenses
Segment Reporting Information [Line Items]
General corporate expenses										103,070	105,740	69,552
Depreciation and amortization										14,738	12,032	14,457
North America
Segment Reporting Information [Line Items]
Net revenue										979,302	1,047,251	910,349
North America \| Operating Segments
Segment Reporting Information [Line Items]
Operating income										281,937	351,599	247,587
Depreciation and amortization										36,643	37,137	39,501
Continental Europe
Segment Reporting Information [Line Items]
Net revenue										293,233	304,325	239,223
Continental Europe \| Operating Segments
Segment Reporting Information [Line Items]
Operating income										95,201	114,708	73,689
Depreciation and amortization										1,709	1,347	1,203
United Kingdom
Segment Reporting Information [Line Items]
Net revenue										94,557	107,072	99,989
United Kingdom \| Operating Segments
Segment Reporting Information [Line Items]
Operating income										9,543	18,814	19,939
Depreciation and amortization										802	1,487	1,205
Other
Segment Reporting Information [Line Items]
Net revenue										46,245	55,473	57,350
Other \| Operating Segments
Segment Reporting Information [Line Items]
Operating income										4,374	9,604	(4,358)
Depreciation and amortization										$ 443	$ 597	$ 626