Aggregate number of common shares reserved for future issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Net debt to EBITDA ratio.

+ References

No definition available.

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Balance Type:	na
Period Type:	duration

- Definition

Share based compensation arrangement by share based payment award equity instruments other than options expected to vest.

+ References

No definition available.

+ Details

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Namespace Prefix:	wtw_
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Stock Plans - Additional Information (Detail) - Performance-based stock unit - shares	1 Months Ended			9 Months Ended	12 Months Ended
	May 31, 2018	Jul. 31, 2017	May 31, 2017	Sep. 29, 2018	Dec. 31, 2016	Dec. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options, granted	81,300	47,900	98,500		289,900
Performance period (years)				3 years
Net debt to EBITDA ratio					450.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock issuable				26,900		106,000
Other than options, shares expected to vest					54,500
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock issuable				161,800		227,200
Other than options, shares expected to vest					272,500