Information About Reportable Segments (Detail) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
	Nov. 29, 2017	Sep. 29, 2018	Dec. 30, 2017	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net revenue		$ 365,765		$ 323,687	$ 1,183,735	$ 994,422
Operating income		118,860		91,378	308,638	217,817
Interest expense		35,506		26,993	107,238	82,227
Other expense, net		881		125	1,978	278
Gain on early extinguishment of debt	$ 10,524	0		0	0	(1,554)
Provision for income taxes		12,374	$ 56,560	19,593	19,580	36,457
Net income		70,099		44,667	179,842	100,409
Net loss attributable to the noncontrolling interest		33		52	122	135
Net income attributable to Weight Watchers International, Inc.		70,132		44,719	179,964	100,544
Depreciation and amortization		13,028		14,208	38,924	42,623
Operating Segments
Segment Reporting Information [Line Items]
Operating income		144,387		110,744	384,129	265,943
Depreciation and amortization		9,999		10,516	30,283	31,888
General corporate expenses
Segment Reporting Information [Line Items]
General corporate expenses		25,527		19,366	75,491	48,126
Depreciation and amortization		3,029		3,692	8,641	10,735
North America
Segment Reporting Information [Line Items]
Net revenue		255,281		223,677	819,007	695,397
North America | Operating Segments
Segment Reporting Information [Line Items]
Operating income		99,513		76,206	273,905	189,397
Depreciation and amortization		9,102		9,764	27,744	29,632
Continental Europe
Segment Reporting Information [Line Items]
Net revenue		72,394		60,665	236,518	179,580
Continental Europe | Operating Segments
Segment Reporting Information [Line Items]
Operating income		36,482		27,097	86,890	54,920
Depreciation and amortization		370		306	989	907
United Kingdom
Segment Reporting Information [Line Items]
Net revenue		25,356		25,473	84,850	75,907
United Kingdom | Operating Segments
Segment Reporting Information [Line Items]
Operating income		5,743		5,968	15,646	15,213
Depreciation and amortization		383		259	1,098	887
Other
Segment Reporting Information [Line Items]
Net revenue		12,734		13,872	43,360	43,538
Other | Operating Segments
Segment Reporting Information [Line Items]
Operating income		2,649		1,473	7,688	6,413
Depreciation and amortization		$ 144		$ 187	$ 452	$ 462