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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income/(loss) attributable to Willis Towers Watson	$ 44	$ (54)	$ 317	$ 323
Basic average number of shares outstanding (shares)	131	134	132	136
Dilutive effect of potentially issuable shares (shares)	1	0	0	1
Diluted average number of shares outstanding (shares)	132	134	132	137
Basic earnings/(loss) per share	$ 0.34	$ (0.40)	$ 2.40	$ 2.38
Dilutive effect of potentially issuable shares (USD per share)	(0.01)	0	(0.01)	(0.02)
Diluted earnings/(loss) per share (USD per share)	$ 0.33	$ (0.40)	$ 2.39	$ 2.36