Condensed Consolidated Statements of Changes in Equity (Unaudited) - USD ($) shares in Thousands, $ in Millions		Total	Shares outstanding [Member]	Ordinary shares and APIC [Member]	Retained earnings [Member]	Treasury shares [Member]	AOCL [Member]	[1]	Total WTW shareholders' equity [Member]	Non-controlling interests [Member]	Redeemable noncontrolling interests [Member]
Equity, beginning balance at Dec. 31, 2017		$ 10,249		$ 10,538	$ 1,104	$ (3)	$ (1,513)		$ 10,126	$ 123
Equity, beginning balance (in shares) at Dec. 31, 2017			132,140
Temporary equity, beginning balance at Dec. 31, 2017	[2]										$ 28
Adoption of New Pronouncement (Accounting Standards Update 2014-09 [Member]) at Dec. 31, 2017		317			317				317
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		221			215				215	6
Net income, total		221
Dividends declared		(79)			(79)				(79)
Other comprehensive income/(loss)		84					83		83	1
Other comprehensive income/(loss), total		84
Issuance of shares under employee stock compensation plans		11		11					11
Issue of shares under employee stock compensation plans (in shares)			277
Share-based compensation and net settlements		3		3					3
Foreign currency translation		(4)		(4)					(4)
Equity, ending balance at Mar. 31, 2018		10,802		10,548	1,557	(3)	(1,430)		10,672	130
Equity, ending balance (in shares) at Mar. 31, 2018			132,417
Temporary equity, ending balance at Mar. 31, 2018	[2]										28
Equity, beginning balance at Dec. 31, 2017		10,249		10,538	1,104	(3)	(1,513)		10,126	123
Equity, beginning balance (in shares) at Dec. 31, 2017			132,140
Temporary equity, beginning balance at Dec. 31, 2017	[2]										28
Adoption of New Pronouncement (Accounting Standards Update 2014-09 [Member]) at Dec. 31, 2017		317			317				317
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income, total		286
Equity, ending balance at Jun. 30, 2018		10,347		10,566	1,270	(3)	(1,605)		10,228	119
Equity, ending balance (in shares) at Jun. 30, 2018			130,730
Temporary equity, ending balance at Jun. 30, 2018	[2]										27
Equity, beginning balance at Mar. 31, 2018		10,802		10,548	1,557	(3)	(1,430)		10,672	130
Equity, beginning balance (in shares) at Mar. 31, 2018			132,417
Temporary equity, beginning balance at Mar. 31, 2018	[2]										28
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased		(269)			(269)				(269)
Shares repurchased (in shares)			(1,768)
Net income		63			58				58	5
Net income, redeemable	[2]										2
Net income, total		65
Dividends declared		(76)			(76)				(76)
Dividends attributable to non-controlling interests		(16)								(16)
Dividends attributable to non-controlling interests, redeemable	[2]										(2)
Other comprehensive income/(loss)		(175)					(175)		(175)
Other comprehensive income/(loss), redeemable	[2]										(1)
Other comprehensive income/(loss), total		(176)
Issuance of shares under employee stock compensation plans		7		7					7
Issue of shares under employee stock compensation plans (in shares)			81
Share-based compensation and net settlements		1		1					1
Foreign currency translation		10		10					10
Equity, ending balance at Jun. 30, 2018		10,347		10,566	1,270	(3)	(1,605)		10,228	119
Equity, ending balance (in shares) at Jun. 30, 2018			130,730
Temporary equity, ending balance at Jun. 30, 2018	[2]										27
Equity, beginning balance at Dec. 31, 2018		9,971		10,615	1,201	(3)	(1,961)		9,852	119
Equity, beginning balance (in shares) at Dec. 31, 2018			128,922
Temporary equity, beginning balance at Dec. 31, 2018	[2]										26
Adoption of New Pronouncement (Accounting Standards Update 2018-02 [Member]) at Dec. 31, 2018					36		(36)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		291			287				287	4
Net income, redeemable	[2]										(2)
Net income, total		293
Dividends declared		(85)			(85)				(85)
Other comprehensive income/(loss)		22					23		23	(1)
Other comprehensive income/(loss), total		22
Issuance of shares under employee stock compensation plans		22		22					22
Issue of shares under employee stock compensation plans (in shares)			289
Share-based compensation and net settlements		(7)		(7)					(7)
Equity, ending balance at Mar. 31, 2019		10,214		10,630	1,439	(3)	(1,974)		10,092	122
Equity, ending balance (in shares) at Mar. 31, 2019			129,211
Temporary equity, ending balance at Mar. 31, 2019	[2]										28
Equity, beginning balance at Dec. 31, 2018		9,971		10,615	1,201	(3)	(1,961)		9,852	119
Equity, beginning balance (in shares) at Dec. 31, 2018			128,922
Temporary equity, beginning balance at Dec. 31, 2018	[2]										26
Adoption of New Pronouncement (Accounting Standards Update 2018-02 [Member]) at Dec. 31, 2018					36		(36)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income, total		442
Equity, ending balance at Jun. 30, 2019		10,204		10,644	1,442	(3)	(1,993)		10,090	114
Equity, ending balance (in shares) at Jun. 30, 2019			128,983
Temporary equity, ending balance at Jun. 30, 2019	[2]										28
Equity, beginning balance at Mar. 31, 2019		10,214		10,630	1,439	(3)	(1,974)		10,092	122
Equity, beginning balance (in shares) at Mar. 31, 2019			129,211
Temporary equity, beginning balance at Mar. 31, 2019	[2]										28
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased		(51)			(51)				(51)
Shares repurchased (in shares)			(280)
Net income		147			138				138	9
Net income, redeemable	[2]										2
Net income, total		149
Dividends declared		(84)			(84)				(84)
Dividends attributable to non-controlling interests		(19)								(19)
Dividends attributable to non-controlling interests, redeemable	[2]										(2)
Other comprehensive income/(loss)		(17)					(19)		(19)	2
Other comprehensive income/(loss), total		(17)
Issuance of shares under employee stock compensation plans		5		5					5
Issue of shares under employee stock compensation plans (in shares)			52
Share-based compensation and net settlements		10		10					10
Foreign currency translation		(1)		(1)					(1)
Equity, ending balance at Jun. 30, 2019		$ 10,204		$ 10,644	$ 1,442	$ (3)	$ (1,993)		$ 10,090	$ 114
Equity, ending balance (in shares) at Jun. 30, 2019			128,983
Temporary equity, ending balance at Jun. 30, 2019	[2]										$ 28

[1] Accumulated other comprehensive loss, net of tax (‘AOCL’).

[2] The non-controlling interest is related to Max Matthiessen Holding AB.