Financial Information for Issuers and Other Guarantor Subsidiaries - Unaudited Condensed Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Revenue	$ 2,048		$ 1,990		$ 4,360	$ 4,282
Costs of providing services
Salaries and benefits	1,278		1,275		2,626	2,652
Other operating expenses	412		406		830	829
Depreciation	59		51		113	100
Amortization	123		140		250	281
Transaction and integration expenses	0		55		6	98
Total costs of providing services	1,872		1,927		3,825	3,960
Income from operations	176		63		535	322
Intercompany (expense)/income	0		0		0	0
Interest expense	(56)		(52)		(110)	(103)
Other income, net	67		63		122	119
(LOSS)/INCOME FROM OPERATIONS BEFORE INCOME TAXES	187		74		547	338
Benefit from/(provision for) income taxes	(38)		(9)		(105)	(52)
Equity account for subsidiaries	0		0		0	0
NET INCOME	149	$ 293	65	$ 221	442	286
Income attributable to non-controlling interests	(11)		(7)		(17)	(13)
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	138		58		425	273
Comprehensive income/(loss) before non-controlling interests	132		(111)		447	194
Comprehensive income attributable to non-controlling interests	(13)		(6)		(18)	(13)
Comprehensive income/(loss) attributable to Willis Towers Watson	119		(117)		429	181
Other guarantors [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	22		52		59	94
Depreciation	0		1		1	2
Amortization	0		0		1	1
Transaction and integration expenses	0		0		0	1
Total costs of providing services	22		53		61	98
Income from operations	(22)		(53)		(61)	(98)
Intercompany (expense)/income	59		97		125	189
Interest expense	0		0		0	0
Other income, net	0		1		0	2
(LOSS)/INCOME FROM OPERATIONS BEFORE INCOME TAXES	37		45		64	93
Benefit from/(provision for) income taxes	(10)		(8)		(14)	(16)
Equity account for subsidiaries	110		26		380	207
NET INCOME	137		63		430	284
Income attributable to non-controlling interests	0		0		0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	137		63		430	284
Comprehensive income/(loss) before non-controlling interests	118		(111)		435	192
Comprehensive income attributable to non-controlling interests	0		0		0	0
Comprehensive income/(loss) attributable to Willis Towers Watson	118		(111)		435	192
Non-guarantors [Member]
Condensed Financial Statements, Captions [Line Items]
Revenue	2,045		1,989		4,335	4,275
Costs of providing services
Salaries and benefits	1,259		1,254		2,591	2,615
Other operating expenses	380		332		753	708
Depreciation	59		50		112	98
Amortization	124		140		250	280
Transaction and integration expenses	(2)		55		4	92
Total costs of providing services	1,820		1,831		3,710	3,793
Income from operations	225		158		625	482
Intercompany (expense)/income	(74)		(118)		(151)	(235)
Interest expense	(1)		(6)		(5)	(12)
Other income, net	67		62		122	117
(LOSS)/INCOME FROM OPERATIONS BEFORE INCOME TAXES	217		96		591	352
Benefit from/(provision for) income taxes	(40)		(9)		(109)	(48)
Equity account for subsidiaries	0		0		0	0
NET INCOME	177		87		482	304
Income attributable to non-controlling interests	(11)		(7)		(17)	(13)
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	166		80		465	291
Comprehensive income/(loss) before non-controlling interests	177		(40)		475	202
Comprehensive income attributable to non-controlling interests	(13)		(6)		(18)	(13)
Comprehensive income/(loss) attributable to Willis Towers Watson	164		(46)		457	189
Reportable Legal Entities [Member] | Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Revenue	0		0		0	0
Costs of providing services
Salaries and benefits	1		0		2	1
Other operating expenses	1		2		2	2
Depreciation	0		0		0	0
Amortization	0		0		0	0
Transaction and integration expenses	0		0		0	0
Total costs of providing services	2		2		4	3
Income from operations	(2)		(2)		(4)	(3)
Intercompany (expense)/income	0		0		0	0
Interest expense	(8)		(8)		(15)	(15)
Other income, net	0		0		0	0
(LOSS)/INCOME FROM OPERATIONS BEFORE INCOME TAXES	(10)		(10)		(19)	(18)
Benefit from/(provision for) income taxes	0		0		0	0
Equity account for subsidiaries	148		68		444	291
NET INCOME	138		58		425	273
Income attributable to non-controlling interests	0		0		0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	138		58		425	273
Comprehensive income/(loss) before non-controlling interests	118		(117)		429	181
Comprehensive income attributable to non-controlling interests	0		0		0	0
Comprehensive income/(loss) attributable to Willis Towers Watson	118		(117)		429	181
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	(1)		0		(1)	0
Transaction and integration expenses	0		0		0	0
Total costs of providing services	(1)		0		(1)	0
Income from operations	1		0		1	0
Intercompany (expense)/income	0		0		0	0
Interest expense	0		0		0	0
Other income, net	0		0		0	0
(LOSS)/INCOME FROM OPERATIONS BEFORE INCOME TAXES	1		0		1	0
Benefit from/(provision for) income taxes	0		0		0	0
Equity account for subsidiaries	(372)		(51)		(1,226)	(590)
NET INCOME	(371)		(51)		(1,225)	(590)
Income attributable to non-controlling interests	0		0		0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	(371)		(51)		(1,225)	(590)
Comprehensive income/(loss) before non-controlling interests	(397)		488		(1,231)	(254)
Comprehensive income attributable to non-controlling interests	0		0		0	0
Comprehensive income/(loss) attributable to Willis Towers Watson	(397)		488		(1,231)	(254)
Willis North America [Member] | Reportable Legal Entities [Member]
Condensed Financial Statements, Captions [Line Items]
Revenue	3		1		25	7
Costs of providing services
Salaries and benefits	18		21		33	36
Other operating expenses	9		20		16	25
Depreciation	0		0		0	0
Amortization	0		0		0	0
Transaction and integration expenses	2		0		2	5
Total costs of providing services	29		41		51	66
Income from operations	(26)		(40)		(26)	(59)
Intercompany (expense)/income	(11)		(9)		(23)	(14)
Interest expense	(23)		(11)		(44)	(22)
Other income, net	0		0		0	0
(LOSS)/INCOME FROM OPERATIONS BEFORE INCOME TAXES	(60)		(60)		(93)	(95)
Benefit from/(provision for) income taxes	12		8		19	13
Equity account for subsidiaries	44		(35)		111	(42)
NET INCOME	(4)		(87)		37	(124)
Income attributable to non-controlling interests	0		0		0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	(4)		(87)		37	(124)
Comprehensive income/(loss) before non-controlling interests	23		(153)		42	(175)
Comprehensive income attributable to non-controlling interests	0		0		0	0
Comprehensive income/(loss) attributable to Willis Towers Watson	23		(153)		42	(175)
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		0		0	0
Depreciation	0		0		0	0
Amortization	0		0		0	0
Transaction and integration expenses	0		0		0	0
Total costs of providing services	0		0		0	0
Income from operations	0		0		0	0
Intercompany (expense)/income	26		30		49	60
Interest expense	(24)		(27)		(46)	(54)
Other income, net	0		0		0	0
(LOSS)/INCOME FROM OPERATIONS BEFORE INCOME TAXES	2		3		3	6
Benefit from/(provision for) income taxes	0		0		(1)	(1)
Equity account for subsidiaries	70		(8)		291	134
NET INCOME	72		(5)		293	139
Income attributable to non-controlling interests	0		0		0	0
NET INCOME/(LOSS) ATTRIBUTABLE TO WILLIS TOWERS WATSON	72		(5)		293	139
Comprehensive income/(loss) before non-controlling interests	93		(178)		297	48
Comprehensive income attributable to non-controlling interests	0		0		0	0
Comprehensive income/(loss) attributable to Willis Towers Watson	$ 93		$ (178)		$ 297	$ 48