| Document and Entity Information | May 12, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | May 12, 2021 |
| Entity Registrant Name | WILLIS TOWERS WATSON PLC |
| Entity Incorporation, State or Country Code | L2 |
| Entity File Number | 001-16503 |
| Entity Tax Identification Number | 98-0352587 |
| Entity Address, Address Line One | c/o Willis Group Limited |
| Entity Address, Address Line Two | 51 Lime Street |
| Entity Address, City or Town | London |
| Entity Address, Postal Zip Code | EC3M 7DQ |
| Entity Address, Country | GB |
| Country Region | 44 |
| City Area Code | 20 |
| Local Phone Number | 3124-6000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Ordinary Shares, nominal value $0.000304635 per share |
| Trading Symbol | WLTW |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001140536 |
