Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Numerator:
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
Denominator:
Weighted average shares of common stock outstanding									67,849	67,188	66,280
Effect of dilutive common stock equivalents									2,171	2,362	3,835
Weighted average diluted common shares outstanding									70,020	69,550	70,115
Earnings per share attributable to WW International, Inc.
Basic	$ 0.18	$ 0.80	$ 0.21	$ (0.09)	$ 0.44	$ 0.70	$ 0.80	$ (0.16)	$ 1.11	$ 1.78	$ 3.38
Diluted	$ 0.18	$ 0.78	$ 0.20	$ (0.09)	$ 0.42	$ 0.68	$ 0.78	$ (0.16)	$ 1.07	$ 1.72	$ 3.19