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]	Retained earnings [Member] Cumulative effect period of adoption adjustment [Member]	Treasury shares [Member]	AOCL [Member]		AOCL [Member] Cumulative effect period of adoption adjustment [Member]	[1]	Total WTW shareholders' equity [Member]	Non-controlling interests [Member]	Redeemable noncontrolling interests [Member]
Equity, beginning balance at Dec. 31, 2018		$ 9,971		$ 10,615	$ 1,201		$ (3)	$ (1,961)	[1]			$ 9,852	$ 119
Equity, beginning balance (Accounting Standards Update 2018-02 [Member]) at Dec. 31, 2018						$ 36				$ (36)
Equity, beginning balance (in shares) at Dec. 31, 2018			128,922
Temporary equity, beginning balance at Dec. 31, 2018	[2]													$ 26
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	[1]			23	(1)
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)	[1]			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)	[1]			9,852	119
Equity, beginning balance (Accounting Standards Update 2018-02 [Member]) at Dec. 31, 2018						$ 36				$ (36)
Equity, beginning balance (in shares) at Dec. 31, 2018			128,922
Temporary equity, beginning balance at Dec. 31, 2018	[2]													26
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)	[1]			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)	[1]			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)	(2)	[2]
Other comprehensive income/(loss)		(17)						(19)	[1]			(19)	2
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)	[1]			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 Dec. 31, 2019		10,369		10,687	1,792		(3)	(2,227)				10,249	120
Equity, beginning balance (in shares) at Dec. 31, 2019			128,690
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		313			305							305	8
Dividends declared		(88)			(88)							(88)
Dividends attributable to non-controlling interests		(1)											(1)
Other comprehensive income/(loss)		(220)						(219)				(219)	(1)
Issuance of shares under employee stock compensation plans		3		3								3
Issue of shares under employee stock compensation plans (in shares)			36
Share-based compensation and net settlements		9		9								9
Foreign currency translation		4		4								4
Equity, ending balance at Mar. 31, 2020		10,389		10,703	2,009		(3)	(2,446)				10,263	126
Equity, ending balance (in shares) at Mar. 31, 2020			128,726
Equity, beginning balance at Dec. 31, 2019		10,369		10,687	1,792		(3)	(2,227)				10,249	120
Equity, beginning balance (in shares) at Dec. 31, 2019			128,690
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income, total		415
Equity, ending balance at Jun. 30, 2020		10,456		10,713	2,015		(3)	(2,390)				10,335	121
Equity, ending balance (in shares) at Jun. 30, 2020			128,763
Equity, beginning balance at Mar. 31, 2020		10,389		10,703	2,009		(3)	(2,446)				10,263	126
Equity, beginning balance (in shares) at Mar. 31, 2020			128,726
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		102			94							94	8
Net income, total		102
Dividends declared		(88)			(88)							(88)
Dividends attributable to non-controlling interests		(12)											(12)
Other comprehensive income/(loss)		56						56				56
Issuance of shares under employee stock compensation plans		2		2								2
Issue of shares under employee stock compensation plans (in shares)			37
Share-based compensation and net settlements		12		12								12
Divestiture of non-controlling interests		(1)											(1)
Other		(3)		(3)								(3)
Foreign currency translation		(1)		(1)								(1)
Equity, ending balance at Jun. 30, 2020		$ 10,456		$ 10,713	$ 2,015		$ (3)	$ (2,390)				$ 10,335	$ 121
Equity, ending balance (in shares) at Jun. 30, 2020			128,763

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

[2] The non-controlling interest was related to Max Matthiessen Holding AB. The remaining amount was purchased during the three months ended December 31, 2019.