Earnings Per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Earnings Per Share [Abstract]
Income from continuing operations	$ 184	$ 919	$ 458	$ 1,582
Less: income attributable to non-controllable interests	(2)	(4)	(10)	(9)
Income from continuing operations attributable to WTW	182	915	448	1,573
Income/(loss) from discontinued operations, net of tax	$ 8	$ (12)	$ (27)	$ 247
Basic average number of shares outstanding (shares)	110	129	113	130
Dilutive effect of potentially issuable shares (shares)	1	0	1	0
Diluted average number of shares outstanding (shares)	111	129	114	130
Basic earnings per share from continuing operations attributable to WTW	$ 1.65	$ 7.10	$ 3.95	$ 12.14
Dilutive effect of potentially issuable shares	0	(0.02)	0	(0.04)
Diluted earnings per share from continuing operations attributable to WTW	1.65	7.08	3.95	12.10
Basic earnings/(loss) per share from discontinued operations, net of tax	0.07	(0.09)	(0.24)	1.90
Dilutive effect of potentially issuable shares	0	0	0	0
Diluted earnings/(loss) per share from discontinued operations, net of tax	$ 0.07	$ (0.09)	$ (0.24)	$ 1.90