Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	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
Numerator:
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
Denominator:
Weighted average shares of common stock outstanding									67,188	66,280	64,329
Effect of dilutive common stock equivalents									2,362	3,835	3,919
Weighted average diluted common shares outstanding									69,550	70,115	68,248
Earnings Per Share attributable to WW International, Inc.
Basic	$ 0.44	$ 0.70	$ 0.80	$ (0.16)	$ 0.65	$ 1.05	$ 1.07	$ 0.60	$ 1.78	$ 3.38	$ 2.54
Diluted	$ 0.42	$ 0.68	$ 0.78	$ (0.16)	$ 0.63	$ 1.00	$ 1.01	$ 0.56	$ 1.72	$ 3.19	$ 2.40