Earnings Per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Earnings Per Share [Abstract]
Income from continuing operations	$ 160	$ 117	$ 274	$ 663
Less: income attributable to non-controllable interests	5	2	8	5
Income from continuing operations attributable to WTW	155	115	266	658
(LOSS)/INCOME FROM DISCONTINUED OPERATIONS, NET OF TAX	$ (46)	$ 69	$ (35)	$ 259
Basic average number of shares outstanding (shares)	112	130	115	130
Dilutive effect of potentially issuable shares (shares)	0	0	0	0
Diluted average number of shares outstanding (shares)	112	130	115	130
Basic earnings per share from continuing operations attributable to WTW	$ 1.38	$ 0.89	$ 2.31	$ 5.07
Dilutive effect of potentially issuable shares	0	(0.01)	0	(0.02)
Diluted earnings per share from continuing operations attributable to WTW	1.38	0.88	2.31	5.05
Basic (loss)/ earnings per share from discontinued operations, net of tax	(0.41)	0.53	(0.30)	1.99
Dilutive effect of potentially issuable shares	0	0	0	0
Diluted earnings per share from discontinued operations, net of tax	$ (0.41)	$ 0.53	$ (0.30)	$ 1.99