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Share-based Compensation - Narrative (Details) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
	Jan. 27, 2026	Sep. 30, 2026	Jun. 30, 2026	Mar. 31, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-Based Payment Arrangement, Expense		$ 70			$ 36	$ 179	$ 104
Shares issued						692,000
Issuance of shares under employee stock compensation plans (in shares)						467,000
Net total number of shares						342,000
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						15,000
Restricted Stock Units [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vested (shares)						569,000
Number of shares, granted						1,129,000
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		3,000			187,000
Restricted Stock Units [Member] \| Not Issued [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vested (shares)						227,000
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		$ 21			$ 0	$ 65	$ 0