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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
NET INCOME ATTRIBUTABLE TO WILLIS TOWERS WATSON	$ 138	$ 58	$ 425	$ 273
Basic average number of shares outstanding (shares)	130	132	130	132
Dilutive effect of potentially issuable shares (shares)	0	1	0	1
Diluted average number of shares outstanding (shares)	130	133	130	133
Basic earnings per share	$ 1.06	$ 0.44	$ 3.27	$ 2.06
Dilutive effect of potentially issuable shares (USD per share)	0	0	(0.01)	(0.01)
Diluted earnings per share	$ 1.06	$ 0.44	$ 3.26	$ 2.05