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Share-based Compensation - Narrative (Details) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
	Jan. 27, 2026	Jun. 30, 2026	Mar. 31, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-Based Payment Arrangement, Expense		$ 67		$ 31	$ 109	$ 68
Shares issued					672,000
Issuance of shares under employee stock compensation plans (in shares)					447,000
Net total number of shares					326,000
Newfront Insurance Holdings, Inc
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Acquisition of Newfront (in shares)		26,353	199,028		225,000
Nonqualified Plan [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Shares issued under ESPP					11,000
Restricted Stock Units [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vested (shares)					526,000
Number of shares, granted					1,112,000
Restricted Stock Units [Member] \| Newfront Insurance Holdings, Inc
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of shares, granted	187,000,000	3,000,000			187,000
Restricted Stock Units [Member] \| Not Issued [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vested (shares)					200,000
Restricted Stock Awards [Member] \| Newfront Insurance Holdings, Inc
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Shares issued under ESPP		26,000	199,000
Number of shares, granted	225,000
Employee Share Purchase Plan [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Shares issued under ESPP					110,000
Location, Statement of Income, Balance [Axis]: wtw:BusinessCombinationIntegrationRelatedCostsCredits
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-Based Payment Arrangement, Expense		$ 35		$ 0	$ 44	$ 0