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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income attributable to Willis Towers Watson	$ 94	$ 138	$ 399	$ 425
Basic average number of shares outstanding (shares)	129	130	130	130
Dilutive effect of potentially issuable shares (shares)	1	0	0	0
Diluted average number of shares outstanding (shares)	130	130	130	130
Basic earnings per share	$ 0.73	$ 1.06	$ 3.08	$ 3.27
Dilutive effect of potentially issuable shares (USD per share)	(0.01)	0	(0.01)	(0.01)
Diluted earnings per share	$ 0.72	$ 1.06	$ 3.07	$ 3.26