Earnings Per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Earnings Per Share [Abstract]
Income from continuing operations	$ 114	$ 546
Less: income attributable to non-controllable interests	3	3
Income from continuing operations attributable to WTW	111	543
Income from discontinued operations, net of tax	$ 11	$ 190
Basic average number of shares outstanding (shares)	118	130
Dilutive effect of potentially issuable shares (shares)	0	0
Diluted average number of shares outstanding (shares)	118	130
Basic earnings per share from continuing operations attributable to WTW	$ 0.94	$ 4.18
Dilutive effect of potentially issuable shares	0	(0.01)
Diluted earnings per share from continuing operations attributable to WTW	0.94	4.17
Basic earnings per share from discontinued operations, net of tax	0.09	1.46
Dilutive effect of potentially issuable shares	0	0
Diluted earnings per share from discontinued operations, net of tax	$ 0.09	$ 1.46