Face (par) amount of debt instrument at time of issuance.

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Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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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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Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Publisher FASB
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Balance Type:	na
Period Type:	duration

- Definition

Gross amount of debt extinguished.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

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Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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-Publisher FASB
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Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow from a borrowing with the highest claim on the assets of the entity in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle, if longer).

+ References

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Balance Type:	debit
Period Type:	duration

- Definition

Debt instrument interest accrual date description.

+ References

No definition available.

+ Details

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Name:	us-gaap_CreditFacilityAxis=wltw_TermLoanFacilityMember
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- Details

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Debt - Narrative (Details) - USD ($)		6 Months Ended
Debt - Narrative (Details) - USD ($)	May 29, 2020	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Sep. 10, 2019
Debt Instrument [Line Items]
Proceeds from senior notes		$ 282,000,000	$ 0
2.950% senior notes due 2029 [Member]
Debt Instrument [Line Items]
Stated interest rate		2.95%		2.95%
2.950% senior notes due 2029 [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 275,000,000				$ 450,000,000
Stated interest rate	2.95%
Effective interest rate (as a percent)	2.697%
Maturity date	Sep. 15, 2029
Description of interest accrual date	Interest accrues on the 2029 senior notes from March 15, 2020 and will be paid in cash on March 15 and September 15 of each year, commencing on September 15, 2020.
Proceeds from debt, net of issuance costs	$ 280,000,000
Proceeds from senior notes	2,000,000
Term Loan Facility [Member]
Debt Instrument [Line Items]
Repayment of outstanding debt	$ 175,000,000
Term loan facility expiration date	Jul. 31, 2020
Revolving Credit Facility [Member] \| Revolving 1.25 Billion Dollar Credit Facility [Member]
Debt Instrument [Line Items]
Repayment of outstanding debt	$ 105,000,000
Maximum borrowing capacity		$ 1,250,000,000