Financial Information for Issuers and Other Guarantor Subsidiaries - Unaudited Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
ASSETS
Cash and cash equivalents	$ 812	$ 1,033		$ 1,033	$ 911		$ 1,030
Fiduciary assets	14,609			12,604
Accounts receivable, net	2,461			2,379
Prepaid and other current assets	451			404
Total current assets	18,333			16,420
Intercompany receivables, net	0			0
Fixed assets, net	967			942
Goodwill	10,454			10,465
Other intangible assets, net	3,055			3,318
Right-of-use assets	930		$ 1,000
Pension benefits assets	886			773
Other non-current assets	541			467
Total non-current assets	16,833			15,965
Investments in subsidiaries	0			0
TOTAL ASSETS	35,166			32,385
LIABILITIES AND EQUITY
Fiduciary liabilities	14,609			12,604
Deferred revenue and accrued expenses	1,444			1,647
Current debt	187			186
Current lease liabilities	154
Other current liabilities	810			864
Total current liabilities	17,204			15,301
Intercompany payables, net	0			0
Long-term debt	4,284			4,389
Liability for pension benefits	1,125			1,170
Deferred tax liabilities	532			559
Provision for liabilities	549			540
Long-term lease liabilities	941
Other non-current liabilities	299			429
Total non-current liabilities	7,730			7,087
TOTAL LIABILITIES	24,934			22,388
REDEEMABLE NON-CONTROLLING INTEREST	28			26
EQUITY
Total Willis Towers Watson shareholders’ equity	10,090			9,852
Non-controlling interests	114			119
Total equity	10,204	10,214		9,971	10,347	$ 10,802	10,249
TOTAL LIABILITIES AND EQUITY	35,166			32,385
Other guarantors [Member]
ASSETS
Cash and cash equivalents	0	0		0	1		1
Fiduciary assets	0			0
Accounts receivable, net	0			0
Prepaid and other current assets	19			33
Total current assets	19			33
Intercompany receivables, net	0			0
Fixed assets, net	17			16
Goodwill	0			0
Other intangible assets, net	56			58
Right-of-use assets	0
Pension benefits assets	0			0
Other non-current assets	48			49
Total non-current assets	121			123
Investments in subsidiaries	8,038			8,108
TOTAL ASSETS	8,178			8,264
LIABILITIES AND EQUITY
Fiduciary liabilities	0			0
Deferred revenue and accrued expenses	0			3
Current debt	0			0
Current lease liabilities	0
Other current liabilities	11			13
Total current liabilities	11			16
Intercompany payables, net	4,754			4,691
Long-term debt	0			0
Liability for pension benefits	0			0
Deferred tax liabilities	0			0
Provision for liabilities	0			0
Long-term lease liabilities	0
Other non-current liabilities	4			5
Total non-current liabilities	4,758			4,696
TOTAL LIABILITIES	4,769			4,712
REDEEMABLE NON-CONTROLLING INTEREST	0			0
EQUITY
Total Willis Towers Watson shareholders’ equity	3,409			3,552
Non-controlling interests	0			0
Total equity	3,409			3,552
TOTAL LIABILITIES AND EQUITY	8,178			8,264
Non-guarantors [Member]
ASSETS
Cash and cash equivalents	812	1,033		1,033	909		1,027
Fiduciary assets	14,609			12,604
Accounts receivable, net	2,434			2,355
Prepaid and other current assets	357			357
Total current assets	18,212			16,349
Intercompany receivables, net	0			0
Fixed assets, net	950			926
Goodwill	10,454			10,465
Other intangible assets, net	3,055			3,318
Right-of-use assets	930
Pension benefits assets	886			773
Other non-current assets	465			452
Total non-current assets	16,740			15,934
Investments in subsidiaries	0			0
TOTAL ASSETS	34,952			32,283
LIABILITIES AND EQUITY
Fiduciary liabilities	14,609			12,604
Deferred revenue and accrued expenses	1,443			1,641
Current debt	0			0
Current lease liabilities	154
Other current liabilities	826			935
Total current liabilities	17,032			15,180
Intercompany payables, net	866			517
Long-term debt	5			0
Liability for pension benefits	1,125			1,170
Deferred tax liabilities	645			688
Provision for liabilities	429			420
Long-term lease liabilities	941
Other non-current liabilities	278			411
Total non-current liabilities	4,289			3,206
TOTAL LIABILITIES	21,321			18,386
REDEEMABLE NON-CONTROLLING INTEREST	28			26
EQUITY
Total Willis Towers Watson shareholders’ equity	13,489			13,752
Non-controlling interests	114			119
Total equity	13,603			13,871
TOTAL LIABILITIES AND EQUITY	34,952			32,283
Reportable Legal Entities [Member] | Parent Company [Member]
ASSETS
Cash and cash equivalents	0	0		0	1		2
Fiduciary assets	0			0
Accounts receivable, net	0			0
Prepaid and other current assets	0			0
Total current assets	0			0
Intercompany receivables, net	5,111			4,755
Fixed assets, net	0			0
Goodwill	0			0
Other intangible assets, net	0			0
Right-of-use assets	0
Pension benefits assets	0			0
Other non-current assets	0			0
Total non-current assets	5,111			4,755
Investments in subsidiaries	5,580			5,691
TOTAL ASSETS	10,691			10,446
LIABILITIES AND EQUITY
Fiduciary liabilities	0			0
Deferred revenue and accrued expenses	0			1
Current debt	0			0
Current lease liabilities	0
Other current liabilities	103			95
Total current liabilities	103			96
Intercompany payables, net	0			0
Long-term debt	498			498
Liability for pension benefits	0			0
Deferred tax liabilities	0			0
Provision for liabilities	0			0
Long-term lease liabilities	0
Other non-current liabilities	0			0
Total non-current liabilities	498			498
TOTAL LIABILITIES	601			594
REDEEMABLE NON-CONTROLLING INTEREST	0			0
EQUITY
Total Willis Towers Watson shareholders’ equity	10,090			9,852
Non-controlling interests	0			0
Total equity	10,090			9,852
TOTAL LIABILITIES AND EQUITY	10,691			10,446
Consolidating adjustments [Member]
ASSETS
Cash and cash equivalents	0	0		0	0		0
Fiduciary assets	0			0
Accounts receivable, net	1			0
Prepaid and other current assets	(241)			(298)
Total current assets	(240)			(298)
Intercompany receivables, net	(6,357)			(6,110)
Fixed assets, net	0			0
Goodwill	0			0
Other intangible assets, net	(56)			(58)
Right-of-use assets	0
Pension benefits assets	0			0
Other non-current assets	(115)			(128)
Total non-current assets	(6,528)			(6,296)
Investments in subsidiaries	(22,527)			(23,125)
TOTAL ASSETS	(29,295)			(29,719)
LIABILITIES AND EQUITY
Fiduciary liabilities	0			0
Deferred revenue and accrued expenses	0			0
Current debt	0			0
Current lease liabilities	0
Other current liabilities	(191)			(250)
Total current liabilities	(191)			(250)
Intercompany payables, net	(6,357)			(6,110)
Long-term debt	0			0
Liability for pension benefits	0			0
Deferred tax liabilities	(113)			(129)
Provision for liabilities	0			0
Long-term lease liabilities	0
Other non-current liabilities	0			0
Total non-current liabilities	(6,470)			(6,239)
TOTAL LIABILITIES	(6,661)			(6,489)
REDEEMABLE NON-CONTROLLING INTEREST	0			0
EQUITY
Total Willis Towers Watson shareholders’ equity	(22,634)			(23,230)
Non-controlling interests	0			0
Total equity	(22,634)			(23,230)
TOTAL LIABILITIES AND EQUITY	(29,295)			(29,719)
Willis North America [Member] | Reportable Legal Entities [Member]
ASSETS
Cash and cash equivalents	0	0		0	0		0
Fiduciary assets	0			0
Accounts receivable, net	26			24
Prepaid and other current assets	316			311
Total current assets	342			335
Intercompany receivables, net	0			0
Fixed assets, net	0			0
Goodwill	0			0
Other intangible assets, net	0			0
Right-of-use assets	0
Pension benefits assets	0			0
Other non-current assets	141			92
Total non-current assets	141			92
Investments in subsidiaries	6,406			6,649
TOTAL ASSETS	6,889			7,076
LIABILITIES AND EQUITY
Fiduciary liabilities	0			0
Deferred revenue and accrued expenses	1			2
Current debt	187			186
Current lease liabilities	0
Other current liabilities	34			38
Total current liabilities	222			226
Intercompany payables, net	737			902
Long-term debt	1,631			1,635
Liability for pension benefits	0			0
Deferred tax liabilities	0			0
Provision for liabilities	120			120
Long-term lease liabilities	0
Other non-current liabilities	17			13
Total non-current liabilities	2,505			2,670
TOTAL LIABILITIES	2,727			2,896
REDEEMABLE NON-CONTROLLING INTEREST	0			0
EQUITY
Total Willis Towers Watson shareholders’ equity	4,162			4,180
Non-controlling interests	0			0
Total equity	4,162			4,180
TOTAL LIABILITIES AND EQUITY	6,889			7,076
Trinity Acquisition plc [Member] | Reportable Legal Entities [Member]
ASSETS
Cash and cash equivalents	0	$ 0		0	$ 0		$ 0
Fiduciary assets	0			0
Accounts receivable, net	0			0
Prepaid and other current assets	0			1
Total current assets	0			1
Intercompany receivables, net	1,246			1,355
Fixed assets, net	0			0
Goodwill	0			0
Other intangible assets, net	0			0
Right-of-use assets	0
Pension benefits assets	0			0
Other non-current assets	2			2
Total non-current assets	1,248			1,357
Investments in subsidiaries	2,503			2,677
TOTAL ASSETS	3,751			4,035
LIABILITIES AND EQUITY
Fiduciary liabilities	0			0
Deferred revenue and accrued expenses	0			0
Current debt	0			0
Current lease liabilities	0
Other current liabilities	27			33
Total current liabilities	27			33
Intercompany payables, net	0			0
Long-term debt	2,150			2,256
Liability for pension benefits	0			0
Deferred tax liabilities	0			0
Provision for liabilities	0			0
Long-term lease liabilities	0
Other non-current liabilities	0			0
Total non-current liabilities	2,150			2,256
TOTAL LIABILITIES	2,177			2,289
REDEEMABLE NON-CONTROLLING INTEREST	0			0
EQUITY
Total Willis Towers Watson shareholders’ equity	1,574			1,746
Non-controlling interests	0			0
Total equity	1,574			1,746
TOTAL LIABILITIES AND EQUITY	$ 3,751			$ 4,035