Revenue - Schedule of Capitalized and Deferred Costs (Details) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
New Accounting Pronouncements And Changes In Accounting Principles [Abstract]
Capitalized Contract Cost, Net	$ 126
Capitalized Contract Cost, Additions	321
Capitalized Contract Cost, Amortization	(327)
Capitalized Contract Cost, Impairment Loss	0
Capitalized Contract Cost, Foreign Exchange Translation Gain (Loss)	(1)
Capitalized Contract Cost, Net	$ 119

Amount of amortization expense for asset recognized from cost incurred to obtain or fulfill contract with customer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

+ Details

Name:	us-gaap_CapitalizedContractCostAmortization
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of impairment loss for asset recognized from cost incurred to obtain or fulfill contract with customer.

+ References

+ Details

Name:	us-gaap_CapitalizedContractCostImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

+ Details

Name:	us-gaap_CapitalizedContractCostNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_NewAccountingPronouncementsAndChangesInAccountingPrinciplesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Capitalized Contract Cost, Additions

+ References

No definition available.

+ Details

Name:	wltw_CapitalizedContractCostAdditions
Namespace Prefix:	wltw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Capitalized Contract Cost, Foreign Exchange Translation Gain (Loss)

+ References

No definition available.

+ Details

Name:	wltw_CapitalizedContractCostForeignExchangeTranslationGainLoss
Namespace Prefix:	wltw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration