Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenue	$ 9,039	$ 8,513	$ 8,202
Costs of providing services
Salaries and benefits	5,249	5,123	4,967
Other operating expenses	1,719	1,637	1,534
Depreciation	240	208	203
Amortization	489	534	581
Restructuring costs	0	0	132	[1]
Transaction and integration expenses	13	202	269
Total costs of providing services	7,710	7,704	7,686
Income from operations	1,329	809	516
Interest expense	(234)	(208)	(188)
Other income, net	227	250	164
INCOME FROM OPERATIONS BEFORE INCOME TAXES	1,322	851	492
(Provision for)/benefit from income taxes	(249)	(136)	100
NET INCOME	1,073	715	592
Income attributable to non-controlling interests	(29)	(20)	(24)
NET INCOME ATTRIBUTABLE TO WILLIS TOWERS WATSON	$ 1,044	$ 695	$ 568
EARNINGS PER SHARE
Basic earnings per share	$ 8.05	$ 5.29	$ 4.21
Diluted earnings per share	$ 8.02	$ 5.27	$ 4.18
NET INCOME	$ 1,073	$ 715	$ 592
Other comprehensive (loss)/income, net of tax:
Foreign currency translation	78	(251)	295
Defined pension and post-retirement benefits	(329)	(199)	14
Derivative instruments	21	2	75
Other comprehensive (loss)/income, net of tax, before non-controlling interests	(230)	(448)	384
Comprehensive income before non-controlling interests	843	267	976
Comprehensive income attributable to non-controlling interests	(29)	(20)	(37)
Comprehensive income/(loss) attributable to Willis Towers Watson	$ 814	$ 247	$ 939

[1] Presented net of a $2 million reversal of an estimated expense related to the Company’s 2016 Business Restructure Program.