Segment and Geographic Data - Information About Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Jan. 02, 2021	Sep. 26, 2020	Jun. 27, 2020	Mar. 28, 2020	Dec. 28, 2019	Sep. 28, 2019	Jun. 29, 2019	Mar. 30, 2019	Jan. 02, 2021	Dec. 28, 2019	Dec. 29, 2018
Segment Reporting Information [Line Items]
Net revenue	$ 323,427	$ 320,699	$ 333,637	$ 400,361	$ 332,583	$ 348,567	$ 369,023	$ 363,164	$ 1,378,124	$ 1,413,337	$ 1,514,121
Operating income	47,668	92,642	50,985	24,867	65,886	94,729	105,473	21,897	216,162	287,985	388,985
Interest expense									123,310	135,267	142,346
Other expense, net									349	1,758	2,578
Provision for income taxes									17,462	31,513	20,493
Net income									75,041	119,447	223,568
Net loss attributable to the noncontrolling interest									38	169	181
Net income attributable to WW International, Inc.	$ 12,611	$ 54,525	$ 14,006	$ (6,063)	$ 29,383	$ 47,086	$ 53,834	$ (10,687)	75,079	119,616	223,749
Depreciation and amortization									59,522	54,335	52,600
Operating Segments
Segment Reporting Information [Line Items]
Operating income									407,460	391,055	494,725
Depreciation and amortization									42,742	39,597	40,568
General corporate expenses
Segment Reporting Information [Line Items]
General corporate expenses									191,298	103,070	105,740
Depreciation and amortization									16,780	14,738	12,032
North America
Segment Reporting Information [Line Items]
Net revenue									942,100	979,302	1,047,251
North America | Operating Segments
Segment Reporting Information [Line Items]
Operating income									269,580	281,937	351,599
Depreciation and amortization									39,740	36,643	37,137
Continental Europe
Segment Reporting Information [Line Items]
Net revenue									313,380	293,233	304,325
Continental Europe | Operating Segments
Segment Reporting Information [Line Items]
Operating income									124,891	95,201	114,708
Depreciation and amortization									1,615	1,709	1,347
United Kingdom
Segment Reporting Information [Line Items]
Net revenue									84,387	94,557	107,072
United Kingdom | Operating Segments
Segment Reporting Information [Line Items]
Operating income									10,648	9,543	18,814
Depreciation and amortization									1,017	802	1,487
Other
Segment Reporting Information [Line Items]
Net revenue									38,257	46,245	55,473
Other | Operating Segments
Segment Reporting Information [Line Items]
Operating income									2,341	4,374	9,604
Depreciation and amortization									$ 370	$ 443	$ 597