Financial Information for Issuers and Other Guarantor Subsidiaries - Unaudited Condensed Consolidated Statement of Comprehensive Income (Details) - 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
Condensed Financial Statements, Captions [Line Items]
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
Intercompany income/(expense)	0			0			0	0
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)
Equity account for subsidiaries	0			0			0	0
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
Comprehensive income/(loss) before non- controlling interests	24			34			218	546
Comprehensive income attributable to non- controlling interests	(2)			12			(15)	(15)
Comprehensive income attributable to Willis Towers Watson	22			46			203	531
Other guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Revenue	1			0			0	0
Costs of providing services
Salaries and benefits	1			0			0	0
Other operating expenses	34			29			128	73
Depreciation	1			2			3	5
Amortization	1			1			2	3
Restructuring costs	0			1			0	5
Transaction and integration expenses	0			2			1	32
Total costs of providing services	37			35			134	118
Income/(loss) from operations	(36)			(35)			(134)	(118)
Intercompany income/(expense)	58			93			247	261
Interest expense	0			0			0	0
Other income, net	0			48			2	48
INCOME/(LOSS) FROM OPERATIONS BEFORE INCOME TAXES	22			106			115	191
Benefit from/(provision for) income taxes	(5)			(10)			(21)	(18)
Equity account for subsidiaries	33			(131)			240	169
NET INCOME/(LOSS)	50			(35)			334	342
Income attributable to non-controlling interests	0			0			0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	50			(35)			334	342
Comprehensive income/(loss) before non- controlling interests	29			62			220	552
Comprehensive income attributable to non- controlling interests	0			0			0	0
Comprehensive income attributable to Willis Towers Watson	29			62			220	552
Non-guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Revenue	1,851			1,849			6,127	6,110
Costs of providing services
Salaries and benefits	1,221			1,190			3,837	3,632
Other operating expenses	332			330			1,040	1,066
Depreciation	52			52			150	146
Amortization	128			143			408	441
Restructuring costs	0			29			0	79
Transaction and integration expenses	51			68			143	139
Total costs of providing services	1,784			1,812			5,578	5,503
Income/(loss) from operations	67			37			549	607
Intercompany income/(expense)	(95)			(121)			(330)	(366)
Interest expense	(3)			(5)			(15)	(15)
Other income, net	70			187			187	264
INCOME/(LOSS) FROM OPERATIONS BEFORE INCOME TAXES	39			98			391	490
Benefit from/(provision for) income taxes	0			(14)			(48)	(63)
Equity account for subsidiaries	0			0			0	0
NET INCOME/(LOSS)	39			84			343	427
Income attributable to non-controlling interests	(2)			0			(15)	(16)
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	37			84			328	411
Comprehensive income/(loss) before non- controlling interests	22			249			223	687
Comprehensive income attributable to non- controlling interests	(2)			12			(15)	(15)
Comprehensive income attributable to Willis Towers Watson	20			261			208	672
Reportable Legal Entities [Member] | Parent Company
Condensed Financial Statements, Captions [Line Items]
Revenue	0			0			0	0
Costs of providing services
Salaries and benefits	1			2			2	4
Other operating expenses	1			0			3	2
Depreciation	0			0			0	0
Amortization	0			0			0	0
Restructuring costs	0			0			0	0
Transaction and integration expenses	0			0			0	0
Total costs of providing services	2			2			5	6
Income/(loss) from operations	(2)			(2)			(5)	(6)
Intercompany income/(expense)	0			0			0	0
Interest expense	(7)			(8)			(22)	(23)
Other income, net	0			0			0	0
INCOME/(LOSS) FROM OPERATIONS BEFORE INCOME TAXES	(9)			(10)			(27)	(29)
Benefit from/(provision for) income taxes	0			1			0	2
Equity account for subsidiaries	53			(45)			344	350
NET INCOME/(LOSS)	44			(54)			317	323
Income attributable to non-controlling interests	0			0			0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	44			(54)			317	323
Comprehensive income/(loss) before non- controlling interests	22			46			203	531
Comprehensive income attributable to non- controlling interests	0			0			0	0
Comprehensive income attributable to Willis Towers Watson	22			46			203	531
Consolidating adjustments [Member]
Condensed Financial Statements, Captions [Line Items]
Revenue	0			0			0	0
Costs of providing services
Salaries and benefits	0			0			0	0
Other operating expenses	0			0			0	0
Depreciation	0			0			0	0
Amortization	(2)			(3)			(2)	(3)
Restructuring costs	0			0			0	0
Transaction and integration expenses	0			0			0	0
Total costs of providing services	(2)			(3)			(2)	(3)
Income/(loss) from operations	2			3			2	3
Intercompany income/(expense)	0			0			0	0
Interest expense	0			0			0	0
Other income, net	0			(197)			0	(197)
INCOME/(LOSS) FROM OPERATIONS BEFORE INCOME TAXES	2			(194)			2	(194)
Benefit from/(provision for) income taxes	0			0			0	0
Equity account for subsidiaries	(56)			532			(646)	(613)
NET INCOME/(LOSS)	(54)			338			(644)	(807)
Income attributable to non-controlling interests	0			0			0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	(54)			338			(644)	(807)
Comprehensive income/(loss) before non- controlling interests	10			(110)			(242)	(1,628)
Comprehensive income attributable to non- controlling interests	0			0			0	0
Comprehensive income attributable to Willis Towers Watson	10			(110)			(242)	(1,628)
Willis North America [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Revenue	7			3			14	14
Costs of providing services
Salaries and benefits	15			20			51	40
Other operating expenses	7			6			32	16
Depreciation	0			0			0	0
Amortization	0			0			0	0
Restructuring costs	0			1			0	1
Transaction and integration expenses	(1)			4			4	6
Total costs of providing services	21			31			87	63
Income/(loss) from operations	(14)			(28)			(73)	(49)
Intercompany income/(expense)	4			(2)			(10)	16
Interest expense	(13)			(9)			(35)	(25)
Other income, net	0			0			0	0
INCOME/(LOSS) FROM OPERATIONS BEFORE INCOME TAXES	(23)			(39)			(118)	(58)
Benefit from/(provision for) income taxes	16			4			29	7
Equity account for subsidiaries	(8)			(248)			(50)	(23)
NET INCOME/(LOSS)	(15)			(283)			(139)	(74)
Income attributable to non-controlling interests	0			0			0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	(15)			(283)			(139)	(74)
Comprehensive income/(loss) before non- controlling interests	(18)			(215)			(193)	68
Comprehensive income attributable to non- controlling interests	0			0			0	0
Comprehensive income attributable to Willis Towers Watson	(18)			(215)			(193)	68
Trinity Acquisition plc [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Revenue	0			0			0	0
Costs of providing services
Salaries and benefits	0			0			0	0
Other operating expenses	0			1			0	1
Depreciation	0			0			0	0
Amortization	0			0			0	0
Restructuring costs	0			0			0	0
Transaction and integration expenses	0			0			0	0
Total costs of providing services	0			1			0	1
Income/(loss) from operations	0			(1)			0	(1)
Intercompany income/(expense)	33			30			93	89
Interest expense	(28)			(25)			(82)	(76)
Other income, net	0			0			0	0
INCOME/(LOSS) FROM OPERATIONS BEFORE INCOME TAXES	5			4			11	12
Benefit from/(provision for) income taxes	(1)			0			(2)	(1)
Equity account for subsidiaries	(22)			(108)			112	117
NET INCOME/(LOSS)	(18)			(104)			121	128
Income attributable to non-controlling interests	0			0			0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	(18)			(104)			121	128
Comprehensive income/(loss) before non- controlling interests	(41)			2			7	336
Comprehensive income attributable to non- controlling interests	0			0			0	0
Comprehensive income attributable to Willis Towers Watson	$ (41)			$ 2			$ 7	$ 336