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Information About Reportable Segments (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Nov. 29, 2017	Dec. 29, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 30, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Net revenue		$ 330,386	$ 365,765	$ 409,747	$ 408,223	$ 312,488	$ 323,687	$ 341,673	$ 329,063	$ 1,514,121	$ 1,306,911	$ 1,164,902
Operating income		80,347	118,860	127,708	62,073	49,488	91,378	96,206	30,233	388,985	267,305	200,811
Interest expense										142,346	112,784	115,160
Other expense, net										2,578	472	1,524
Early extinguishment of debt, net	$ 10,524									0	8,969	0
Provision for (benefit from) income taxes										20,493	(18,237)	16,634
Net income		$ 43,785	$ 70,132	$ 70,720	$ 39,112	$ 62,969	$ 44,719	$ 45,173	$ 10,653	223,568	163,317	67,493
Net loss attributable to the noncontrolling interest										181	197	206
Net income attributable to Weight Watchers International, Inc.										223,749	163,514	67,699
Depreciation and amortization										52,600	56,992	58,749
Operating Segments
Segment Reporting Information [Line Items]
Operating income										494,725	336,857	249,398
Depreciation and amortization										40,568	42,535	45,125
General corporate expenses
Segment Reporting Information [Line Items]
General corporate expenses										105,740	69,552	48,587
Depreciation and amortization										12,032	14,457	13,624
North America
Segment Reporting Information [Line Items]
Net revenue										1,047,251	910,349	798,827
North America \| Operating Segments
Segment Reporting Information [Line Items]
Operating income										351,599	247,587	175,290
Depreciation and amortization										37,137	39,501	41,718
United Kingdom
Segment Reporting Information [Line Items]
Net revenue										107,072	99,989	100,808
United Kingdom \| Operating Segments
Segment Reporting Information [Line Items]
Operating income										18,814	19,939	14,199
Depreciation and amortization										1,487	1,205	971
Continental Europe
Segment Reporting Information [Line Items]
Net revenue										304,325	239,223	210,590
Continental Europe \| Operating Segments
Segment Reporting Information [Line Items]
Operating income										114,708	73,689	51,096
Depreciation and amortization										1,347	1,203	1,621
Other
Segment Reporting Information [Line Items]
Net revenue										55,473	57,350	54,677
Other \| Operating Segments
Segment Reporting Information [Line Items]
Operating income										9,604	(4,358)	8,813
Depreciation and amortization										$ 597	$ 626	$ 815