Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenue	$ 1,859			$ 1,852			$ 6,141	$ 6,124
Costs of providing services
Salaries and benefits	1,238			1,212			3,890	3,676
Other operating expenses	374			366			1,203	1,158
Depreciation	53			54			153	151
Amortization	127			141			408	441
Restructuring costs	0			31			0	85
Transaction and integration expenses	50			74			148	177
Total costs of providing services	1,842			1,878			5,802	5,688
Income/(loss) from operations	17			(26)			339	436
Interest expense	(51)			(47)			(154)	(139)
Other income, net	70			38			189	115
INCOME/(LOSS) FROM OPERATIONS BEFORE INCOME TAXES	36			(35)			374	412
Benefit from/(provision for) income taxes	10			(19)			(42)	(73)
NET INCOME/(LOSS)	46	$ 65	$ 221	(54)	$ 41	$ 352	332	339
Income attributable to non-controlling interests	(2)			0			(15)	(16)
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	$ 44			$ (54)			$ 317	$ 323
EARNINGS PER SHARE
Basic earnings/(loss) per share	$ 0.34			$ (0.40)			$ 2.40	$ 2.38
Diluted earnings/(loss) per share	$ 0.33			$ (0.40)			$ 2.39	$ 2.36
Comprehensive income before non-controlling interests	$ 24			$ 34			$ 218	$ 546
Comprehensive (income)/loss attributable to non-controlling interests	(2)			12			(15)	(15)
Comprehensive income attributable to Willis Towers Watson	$ 22			$ 46			$ 203	$ 531