| Component: (Network and Table) | |
|---|---|
| Network | 2434413 - Disclosure - Retirement Benefits - Significant Unobservable Input Reconciliation (Details) (http://www.willistowerswatson.com/role/RetirementBenefitsSignificantUnobservableInputReconciliationDetails) |
| Table | Schedule of Defined Benefit Plans Disclosures [Table] |
| Reporting Entity [Axis] | 0001140536 (http://www.sec.gov/CIK) |
| Defined Benefit Plan Disclosure [Line Items] | Period [Axis] | |
|---|---|---|
2016-01-01 - 2016-12-31 | ||
| Fair Value, Hierarchy [Axis] | ||
| Level 3 | Fair Value Hierarchy [Domain] | |
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] | ||
Fair value of plan assets, beginning of year | 1,000,000 | 4,385,000,000 |
Unrealized and realized gains relating to instruments still held at end of year | ||
Foreign exchange | ||
Fair value of plan assets, end of year | 17,000,000 | 8,109,000,000 |