Acquisitions - Narrative (Details) shares in Thousands, £ in Millions, $ in Millions					1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2026 USD ($)	Apr. 30, 2026 GBP (£)	Jan. 27, 2026 USD ($) shares	May 02, 2025 USD ($)	Dec. 31, 2025 USD ($)	Jun. 30, 2026 shares	Mar. 31, 2026 shares	Jun. 30, 2026 USD ($) shares	Jun. 30, 2025 USD ($)
Business Combination [Line Items]
Acquisition,contingent consideration								$ 6
Acquisitions, cash payment								23	$ 8
Preliminary values of intangible assets								409
Customer Relationships [Member]
Business Combination [Line Items]
Preliminary values of intangible assets								$ 271
Restricted Stock Units [Member]
Business Combination [Line Items]
Number of shares issued \| shares								1,112
Newfront Insurance Holdings, Inc
Business Combination [Line Items]
Acquisitions, cash payment			$ 900
Acquisiiton , equity payment			150
Newfront Insurance Holdings, Inc \| Customer Relationships [Member]
Business Combination [Line Items]
Preliminary values of intangible assets			$ 209
Weighted-average expected lives of intangible assets			13 years
Newfront Insurance Holdings, Inc \| Developed Software [Member]
Business Combination [Line Items]
Preliminary values of intangible assets			$ 55
Weighted-average expected lives of intangible assets			7 years
Newfront Insurance Holdings, Inc \| Performance Targets
Business Combination [Line Items]
Contingent consideration, amount			$ 250
Newfront Insurance Holdings, Inc \| Restricted Stock Units [Member]
Business Combination [Line Items]
Number of shares issued \| shares			187,000			3,000		187
Newfront Insurance Holdings, Inc \| Restricted Stock Awards [Member]
Business Combination [Line Items]
Number of shares issued \| shares			225
Number of ordinary shares vested \| shares						26	199
Newfront Insurance Holdings, Inc \| Senior Notes
Business Combination [Line Items]
Proceeds from debt, net of issuance costs					$ 1,000
Newfront Insurance Holdings, Inc \| Up-Front
Business Combination [Line Items]
Contingent consideration fair value			$ 1,050
Pending Acquisitions \| Al-Futtaim Willis
Business Combination [Line Items]
Acquisitions, cash payment				$ 58
Reamining controlling interest				51.00%
Cushon Acquisition
Business Combination [Line Items]
Acquisitions, cash payment \| £		£ 150
Contingent consideration, amount \| £		£ 100
Cushon Acquisition \| Customer Relationships [Member]
Business Combination [Line Items]
Preliminary values of intangible assets	$ 58
Weighted-average expected lives of intangible assets	20 years	20 years
Cushon Acquisition \| Developed Software [Member]
Business Combination [Line Items]
Preliminary values of intangible assets	$ 49
Weighted-average expected lives of intangible assets	5 years	5 years
Cushon Acquisition \| Trade Names [Member]
Business Combination [Line Items]
Preliminary values of intangible assets	$ 4
Weighted-average expected lives of intangible assets	10 years	10 years
Other [Member]
Business Combination [Line Items]
Acquisitions, cash payment								$ 42
Equity Based Payment \| Newfront Insurance Holdings, Inc \| Above Target Performance
Business Combination [Line Items]
Contingent consideration, amount			$ 150

WILLIS TOWERS WATSON PLC 10-Q